A ganker is someone that preys on "easy" targets, most of the time the ganker is of a higher level or having superior gear available to him/her. Users playing on Normal/RP servers do not have to worry about the above though, unless they quest while being PvP flagged. The ganker will usually prey on easy targets, i.e. people questing in low level zones or attack you while you're otherwise engaged with a mob.
Many gankers are at level 60+ with flying mounts, and a popular tactic is to swoop in unexpectedly, giving the victim almost no time to react. They also consist mainly of Rogues, and to some extent Druids, seeing that they can hide in stealth and await the perfect moment to get an easy PK.
Most people tend to frown upon the gankers, and will generally not attack grey levelled players (-9 levels below themselves) if not provoked.
Please consider the following before you decide to be a ganker. Gankers are extremely unpopular in World of Warcraft and MMO's in general. While you may enjoy ganking, you should always bear in mind your enjoyment comes at the expense of the other player, and should consider carefully if you wish to ruin another persons game time. There is also the risk of said player logging out and coming back for revenge with a stronger alt character or their main as many players have multiple high level characters.