Banning someone from a server means that the player is no longer allowed to join that server and play. Kicking, or sometimes booting, is when you force someone to leave the server, but they may reconnect. Slapping acknowledges that a player did something wrong, but it is often used humorously as well, and simply makes a slapping noise and throws the player off course (without kicking or banning them).
Getting Slapped Edit
If you break any of the rules of the server and it's your first offense, the administrators of the server will usually slap you for breaking the rule. Being slapped makes a loud slapping noise, you get knocked around, and the screen says "<name> was SLAPPED by the Admin!" For all players to see. This is done to show the person and other players that what they have done wrong, and to warn them not to do it again without resorting to kicking them from the server. Usually, this is for breaking a communication rule, like no recruiting for clans in a server.
Sometimes, when multiple admins are on a server, they might slap each other off tall places so the slapped person falls off and does "The lemming thing."
Getting Kicked Edit
Getting kicked (or booted) from a server is usually because of a minor offense; if you break a rule, team kill, or even annoy an administrator you can get booted from a server. Booting holds no permanent effect, as you can come back into the server after getting booted from it. Even though booting isn't major, if you get booted from a server repeatedly, you should reconsider your behavior. Banning is usually the next step.
Getting Banned Edit
Getting banned is a different from getting kicked or slapped in that it is permanent (unless an admin decides to unban you). Banned means that the server blocks you from playing in it by blocking your IP address. Because your Urban Terror name can change, servers record you by your IP address. Getting banned may mean you are blocked from just one server, or in some clans, getting banned in every server that the clan controls. In the case of a ban by the built-in anti-cheat, you will not be able to join any Urban Terror server.
Getting Booted from a Clan Edit
Cheating via aimbots/mombots usually gets you banned from a server, and in many cases, banned from any clan that you might be in. In most cases, you will be unable to join a new clan because you were cheating. Most players and clans don't want to be associated with cheaters.