Alt is an abbreviation for alternate. Virtually all MMORPGs permit a player to create more than one player character on his or her account. An alt is a character to which a person devotes less time and effort than the person's main character.
What constitutes "less" time and effort can be subjective with respect to the player. For most players, the main is the most powerful, highest-level, or simply the oldest character on the account. A few players may play multiple characters simultaneously, keeping them roughly equal in power. In those cases, players often simply use the term alt to refer to a character other than the one they are currently playing.
Alts are created for a variety of reasons.
Some players create alts to have a different social experience. In some cases, the change is minor, being simply a second character with a slightly different personality and/or abilities. In other cases it can be drastic, as in being a member of a different race or gender. Alts may have wholly different circles of friends from the main, or no friends at all.
Alts on the same account cannot both be present at once.
So alts on the same account cannot trade with each other using the trade window.
Separate accounts Edit
There is no restriction on the number of accounts you can have, but each costs an account fee per month.
Alts on separate accounts circumvent the restrictions placed on Alts on the same account, and also lose the benefits. Few players run a separate account just to occasionally trade items.