The Sims 3/Relationships

There are three basic concepts to relationships in the Sims 3. Long-term relationships, short context relationships and commodities.
 * Long-Term Relationship: Describes the way two Sims view each other at a given moment. The long-term relationship starts when two Sims first socialize with one another. The long-term relationship is measured by the green bar below the Sim you are talking to in the upper left hand corner of the screen during a conversation. When a relationship is positive the bar fills up with green. If the relationship isn't doing too good, the green bar will empty out as the conversation progresses and can even sink into the red.

Visitor Privileges
Visitor Privileges are things a Sim can do on another Sim's lot. At "1", Sims can do basic things like eat food that is offered to them, clean dishes and use the bathroom. As the number increases, there are more visitor privileges available.

Long-Term Relationships

 * Short-Term Context: Short-Term Context is what a Sim thinks about another Sim during a conversation. Short-Term Context is in the upper left hand corner of the screen when two Sims are having a conversation. Example "John thinks Paul is being sociable." or "Mary thinks Peter is being friendly." Here is a list of the short-term contexts : Awkward, Amusing, Creepy, Drab, Dull, Friendly, Frightening, Funny, Hilarious, Hot, Impolite, Insufferably Tedious, Insulting, Odd, OK, Seductive, Steamed, Unforgivably Rude, Very Awkward, Very Friendly, Very Scary.
 * Relationship Decay: Relationships between Sims decay every day. Sims need to keep in touch to avoid relationships becoming too low.
 * Charisma Skill: Sims with the Charisma skill that have made atleast 20 friendships have the advantage of no relationship decay.

Friendship
To build up good relationships with Sims, call them on the phone and invite them over. Using a lot of "Friendly" social options and socials directly related to positive traits a Sim has (such as the Ambitious trait) are a good way to boost a friendship.
 * Some Sims want to have long-term relationships with other Sims. (Good friends, best friends, get engaged; etc.) Having friends results in opportunities and positive moodlets. Positive moodlets are the key to earning Lifetime Rewards.
 * Social is a need unless your Sim has the Loner trait.
 * Chatting with co-workers outside of work can lead to promotions.

Romance
Two Sims must have a high enough long-term relationship in order to be romantic with each other. The best way to build a romance is to work on a friendship and when it is high enough start using Romantic socials. With enough romantic and friendly socials, your Sims will soon have the options to go steady, woo-hoo, get engaged, and eventually get married.
 * Sims with the Commitment Issues trait will react poorly to romantic situations.
 * Sims with the Unflirty trait don't like romantic situations and it is difficult for them to love.
 * If a romantic social is rejected, the advancing Sim will get the "Awkward" short-term context. Persistent rejected romantic advances will lead to "Odd" or "Creepy" short-term context. These short-term contexts have an effect on the long-term relationship, meaning the long-term relationship will take a hit. To remedy this, try using Friendly or Amusing socials to turn the conversation around.

Getting Serious and Getting Married
When two Sims have a high enough long-term relationship, options to take the relationship to the next level will become available. An accepted "Propose Going Steady" when the relationship is Hot for both Sims will result in the Sims becoming partners. From there the two Sims can move to Fiancé by using the "Propose Marriage" social. Once engaged the Sims can start planning the wedding if they want to. To get married use the "Get Married" social at a wedding party or just use the "Have Private Marriage" social to enjoy a small ceremony.
 * Sims with the Commitment Issues trait need a very high long-term relationship to get married.
 * Sims with the Insane trait will have random reactions to getting married.

Breaking Up
Breaking up depends on the current status of the long-term relationship. If two Sims are married, you have to sink the relationship with negative Short-Term Contexts. The Sim(s) will need to be mean, rude, and insulting. Once the relationship is low enough, the "Divorce" social will become an option. If two Sim are engaged and sink the relationship, the "Break Up" social will become available.
 * In the Sims 3 there is also the option to have a Sim or Sims Move Out which could be helpful after a nasty break up. Once a Sim moves out, you can play the current active household or change the active household to play the Sim(s) who have moved out.