You might be inclined to jump right into the game, but if you don't know much about it, or have trouble identifying what is what, this page outlines some of the basics.
User interface/HUD
The game's HUD includes your health and mana pools, hot bars, and assigned mouse hotkeys. Additional icons appear above the main display (at the bottom of the screen, in the center) for easy access to additional menus such as the inventory and skills menus.
At the top right of the screen you will see the minimap, which is a transparent box where the walls of the dungeon are drawn so that you can find your bearings and keep track of where you've been and where you have left to explore.
On the top left is where information on your pet's health, mana, skills, status, and items are located.
Fighting
To attack, press 
while holding the cursor over an enemy or breakable (like a crate or vase). Your character will automatically turn towards the target and adjust its range to accommodate for the currently equipped weapon (applies to bare hands and feet as well) before attacking the target. If you hold Shift and together, you will cause your character to attack in the direction of the cursor, without moving (this is a useful technique, primarily used by ranged classes, for guaranteeing that you don't run into danger).
Targeting
When you want to target an object or enemy, just place the cursor over it. The object you've targeted will be surrounded in a red glow to help distinguish it from other objects. Using an attack or an ability on a target will either move your character closer towards it, or use the ability from long range.
Enemies
There are monsters in the game that will try to attack you (some may be more or less aggressive or evasive than others). When you kill a monster, you'll receive experience instantaneously and have a chance to receive loot (gear, items, or money). Loot quality varies, and you'll always have a chance to get at least one drop from an enemy (enemies can drop more than piece of loot at a time).
Statistics
Details on character statistics are unknown at this time.
Items
There are consumable items in Torchlight that can be used to restore your health or mana. These come in the form of potions (red for health, blue for mana).
Pets
You will get to choose a pet from several choices at some point during the game. This companion will help you fight and carry items. You can see an additional area of the HUD (at the top left of the screen) where your pet's health, mana, skills, status, and items are located.
Fishing
Fishing has been announced as a way to give players some alternatives during the game. Through fishing, players can catch beneficial items (probably consumable fish and random treasure). Additional mini-games and professions have not been announced.
Items
Throughout the game you will acquire items. Be it wondrous gear looted from the corpse of an enemy, or several potions purchased from a merchant to keep you alive, you will focus on what you have, what you need, and where to get it all. You can acquire items from shop vendors, killing enemies, destroying objects, and opening treasure chests.
- Gear
The equipment that you acquire will be used to upgrade your character's statistics to make them perform better (survive longer, deal more damage, etc.).
- Potions
Potions are one time consumables that replenish either your health or your mana. You can place potions on the action bar in the HUD to quickly access them with one of the keyboard shortcuts ( 1 or 2 ).