The admin is the player in charge of controlling the server. He/She is, or has been given a password by, the server owner and is entrusted with the responsibility of ensuring fair, fun play in the server. If an admin is active, his or her name will be highlighted in red on the scoreboard. The admin is able to perform several actions, such as switching gamemodes and maps, kicking and banning players, changing the master mode, using forceteam to switch a player's team and several other commands, without the approval of other players. This is because, in the AC voting system, an admin has the deciding vote.
A player sets him/herself as admin by issuing the command "/setadmin 1 password". If the player types a password that is not recognised by the server, their client is kicked. Typos and the possibility of 'saying' the password to the HUD (by omitting the '/') make binding server passwords to keys or scripts a better alternative. Claiming admin will remove the status from an existing admin.
Becoming an Admin
Server owners can set one or more admin passwords in the server configuration. Passwords can be given to those players, or entire clans, that the server owner trusts. It's common for a clan that runs its own server/s to give admin passwords to all its clan members. There are also 'server share' schemes, where server owners agree to share admin passwords amongst each other to ensure a large admin availability on frequently used servers. If you wish to obtain an admin password for a server, speak to the owner or clan via their forum, email, in game or in IRC.
If a player with admin status is behaving badly, eg. changing map frequently, abusing/insulting, kicking/banning without reason, etc., leave the server. There is no point in trying to reason with, or compete with, a rogue admin. If you can, get a demo or screenshot and report the abuse to the server owner.