1. Items
There are many types of items like armor, food, water, elexir or other objects. Basicly each item has an item level from the game options you can turn on to show its level. According to his level item can be poor quality that are the most weakest items or be common, uncommon, rare, epic, legendary and heirlooms. You can understand what is the item quality by his name color the most weakest start from white then gray, light green, dark green, blue, purple and orange. Some itmes can be gived to other players and some can't. Below the name of each item writes does it is soulbound which means that can't be traded or bind when equiped which means that if you equip the item to your character it becomes soulbound. Binds when equiped is relevant only to armor, all other items can be traded.
2. Armor
The basic line of the game is to get a good armor, you can do this by killing boses along with other players. Each class need armor with diffrent stats.
Death Knight, on frost talents for dps, need to get gear that gives: strength, critical strike, hit rating, haste rating. You don't need too much haste or hit the most important stat for you is strength and then crit rating. But if you don't have any hit you willl miss alot, a good hit rating for dk is 320.
If you are a tank no matter warrior, dk or paladin you need items with a lot of stamina, defense rating, dodge, parry and block rating. The max deffense rating that you need is 580. Only death knights don't need block rating, because they can't wear shield.
If you are a druid tank (on feral talents with bear form), cause you can't wear plate items that gives dodge and parry you have to get as much stamina as you can. For that better be with pvp items.
For a dps warrior the armor penetration is the most important stat. You will also need 180 hit rating and a lot critical strike rating. Ofcourse all your items need to be with strength and stamina.
Rogues, feral dps druids and enchancment shamans need much agility, critical strike rating, hit rating, haste rating and armor penetration. You need 380 hit rating (200 for a druid), and for other stats better choose critical strike rating than haste and armor pene.
Paladins and death knights (on unholy or blood dps talents) need strength, crit rating and hit.
You need 180 hit rating and everything else stack with strength and crit.
A healer paladin needs a lot intelect, spell power and mana regen.
A healer druid or priest needs a lot spirit, haste and spell power.
A restoration shaman needs mana regen, spell power and haste.
Every dps caster like mage, lock, shadow priest... needs spell power and crit/haste. For some cases one can be better than other but both are eavenly good.
3. Gems And Enchants
Gems are magical items that increase your attributes they can be putted unto armor to work. Mostly an epic gear has sockets (there gems are putted) that are shown in its tooltip. If it has press shift+right click on the item and a panel will be shown. Here you can put your gems. You can buy gems from other player that have learned the jewelcrafting profession. The gems has colour along with the socket if you put the same colour gems like all the sockets on one item you get socket bonus on your stats. Somethimes its usefull but mostly its not. Just put all your gems to increase your primary stat.
Enchants can be done to an armor. To do this you need to find a player with enchanting profession.