The Essence of Clans
The general meaning of the word Clan in Runescape tends to be one of a group of players united in a common goal or bond that serves to see them unite to achieve anything typically for enjoyment. There are various types of clans from small to large and many people have different opinions on what consists of a clan.
Typically the most accepted terminology of a clan is that it is one of more than three players operating under a common goal. Typically the bigger the clan the more likely it is to be accepted that they are indeed a full-fledged clan and not just a "group of buddies" hanging out together. Essentially, many clans tend to start off just as a group of friends working with each other for enjoyment, company or any other reason that will officially become a clan when more and more people unite typically under one of the persons in the group.
Some clans form "Alliances", these alliances consists of different types of clans. Runescape's oldest and biggest alliance called "The Chaos Alliance" has over 9000 members and not only has clans in it, but teams, clubs, companies, cults, legions and armies, it is led by Red Echidna0. They accept all levels and all clans however they are mainly high leveled experts. Others have made similar alliances, mainly those who believe in strength in numbers.
It is not far-fetched to say that some clans end up forming very much like in fictional stories where people are drawn to others and soon unite to work under one banner. Of course, there are various other ways clans can start up. One prominent way is through internet forums and communities, many people who post on message boards may start up a rapport socially with other members and soon find that they have many things in common.
It could be said then that the essence of a clan is that its members should ideally have something in common else, it is likely to stumble at the first step.
Of the types of clans out there, the list would be innumerable, however you could broadly categorize them into two separate categories:
 Player Killing Clans
Player Killing clans are typically members (pure or impure) who unite together with the common goal of wreaking plenty of havoc in the wilderness where player-killing is quite welcome! Members of raider clans tend to be very similar in levels but have a wide range of combat abilities available to them allowing them to adjust to any combat situation in the wild.
Wars are a common eruption in the wilderness; clans can quite easily fall out of favour with one another resulting in a pitched battle in the wild, usually under an agreed set of rules prior to the date of the "war". No mercy tends to be shown by pking clans regardless of your relationship with them, although there are some who adhere to a set of rules almost akin to a "honour code" which, if broken results in severe repercussions.
The most common rule of this "honour" code tends to be one of not interrupting other wars, or even calling alliances into effect during a war when it had been agreed that no allies would be brought to the battlefield. Indeed many clans have a reputation for breaking the "unspoken" rules so to say although this isn't always detrimental to the clan itself.
Players joining a pking clan are well advised to do their research on the clan itself before even considering joining. An unorganized pking clan is bound to cost its members more money in the long term that it's worth, while an organized one can be quite an enjoyable experience. Watch the member count as well, high amounts of members in a clan may mean your needs will be ignored as the more "loyal" members of the clan are attended to first. It may be better to join a small close-knitted clan where you can get all the help you need without the feeling of being shuffled to one side as cannon-fodder or indeed without feeling like a burden on the more elite members.
 Non-player killing Clans
Non-player killing clans are extremely rare and are incredibly difficult to establish and work efficiently. This could be attributed to lack of enjoyment indeed the lack of combat probably does in fact bother some players but it would be more realistic to say that NPK clans tend not to succeed as much as usual because there's practically no money gain in the short term.
NPK clans tend to operate under a co-operative skill leveling structure. This means some people will work on a supply skill while others will work on a production skill (i.e. woodcutting/fletching) with the supplier giving away everything he gains to the producer who then levels his production skill and either sells his product and gives the money back to the clan or gives all the product back to the clan's banker.
Since in the long term it's more worthwhile to actually stockpile your supplies while training a supply skill and then using it yourself to level up a production skill it is quite easy to see why many people are put off by the idea of an NPK Clan. However you can and do see this method of working-relationship happen every day in Runescape but not under a clan. Rather you will find this happening a lot between friends who may be helping each other out or working on leveling their respective skills so they can pool the cash and buy some valuable item for the two of them to either share or in fact to buy TWO valuable items.
There is another aspect of the NPK clan, which is also quite rare but has much more chance of succeeding than a skill-based clan. Community clans are where groups of people band together to relish in each other's company either in game or on the forums, sharing advice and stories and showing their progress. Community clans tend to be extremely closed knitted and very friendly with members who are extremely willing to help out when they can and as such are very vocal in their support of each other.
Many community clans however don't really broadcast the fact they are a clan rather preferring to remain hidden which may be a reason why very few exist. I theorize this is because they'd rather not have random applicants knocking on the membership door. Of the community clans in Runescape many tend to be "invitation-only" types of clans that require verification and a two-vote policy to even get in. The policies are there apparently to keep the community just as it is, some things after all are considered best left alone and disruptive newcomers aren't always a welcome sight!
 Mixed Clans
There are of course, mixed clans who mix up a bit of skill-building and community with Player-killing. These tend to be the most common clans around and typically like pking clans tend to require certain levels before you can be admitted.
However, most groups of people who join together either choose between what they want their group to be before even recruiting.
Groups of people who unite with one another to complete quests, survive and kill in the Wilderness, or specialize in training particular skills. A clan is family in a sense.
Teams consist of people who follow one particular event, such as
- Kalphite Queen Trips
- King Black Dragon Trips
- Chaos Elemental Trips
- Waterbirth Island Trips
- TzHaar Fight Pit Trips
- Wilderness Bronze Wars
These are only an example of events teams like to follow. There are many other possible things they could focus on.
Usually people who dedicate their Runescape life to following a certain person, creature or God, you get a glimpse of a NPC Cult in the Hazeel's Cult Quest.
Usually communities set up by one person who camp in a certain location.
 Companies and clubs
Companies are usually formed to make money, as in the real world, however some non-profit organizations exist. They vary from guides who help players around the world of Runescape, to hunters who track and kill players down for a price.
 Armies and Empires
These are made for wars, very few are player made, they have similar clan events but mainly focus on training for combat.
 Clan Structures
Clan structures vary throughout the game depending on the founding of the clan. There are "dictatorship" clans led entirely by one person with his close friends as his able generals enforcing the rules and making sure the clan is generally a good/bad place to be! There are some run completely by a democratic voting system which takes place every 6 months electing a new leader to keep things fresh and some that are run by a "council" of prestigious members who dictate where the clan shall go.
Clan structuring is one of the most difficult things to get right and is typically the reason why clans fail at the first hurdle. The difficulty in balancing tyranny with benevolence if you would like, is one that requires trial and error. You CAN be too harsh, too lenient, too easy going, too formal and so on. Also it is quite easy for the leadership and the clan members to have different viewpoints on where the clan should be going, many people allow success to get to their heads, many feel disillusioned either because they're being ignored or because they disagree with events that have been occurring.
There is no right or wrong way to create or structure a clan, however the best advice that could be given is to actually get to know every single person you admit to the clan. There's no point accepting everyone and anyone because chances are they'll all eventually have different goals and ideals and this can split a clan apart. There have been many incidents where high and low profile clans have suddenly found a core group of members splitting away and forming their own clans. Others can simply just "stop" working, be it time constraints or weak leadership or even weak membership. People can drift away and the ideals of the clan long forgotten.
Either way, becoming a member of a clan can be either an enjoyable experience or a downright horrible one. Everyone is advised to try it out once because you simply cannot really be told how a clan is until you actually try several and see if the "clan-life" suits you or not.