The admin for a server typically represents the owners and financiers of the server. Alternatively, an owner can grant administrator rights to a regular player (or clan member) on the server
Admins often have access to functions not usually available to regular players, such as the ability to enter noclip or god mode, slay, burn, blind or slap players and temporarily or permanently ban players. In just about every game mainly FPS and Strategy games, the Admins have the ability to kick and ban but in more modern games such as Crysis, GMOD and Call of Duty they have the ability to do much more as listed above, gimping, burning and slapping. However those forms due not usualy do any harm to the said player, admins often use it to have fun with players that are annoying or muting and gimping spammers
Of course the admin's job is to also regulate the server from players who will break the server rules set by the admins or the owner. Of course there are also those admins who misuse their power and are accused of "abusing" their admin powers by other players who have been the victim of the commands.
The status of admin comes with the responsibility of maintaining fairness and enforcing rules on the server. Administrators' tasks include making the server safe.