Every player has an alignment. You start out as neutral. As you kill
monsters your alignment will change depending on which monsters you decide
to fight. If you kill the bad ones, like the troll, orcs, giants, etc.,
then you will become honorable, then good, then a crusader, then a paladin,
then a white lord. If you kill the good ones, like the frog, beggar, guard,
etc. then you will become malicious, then evil, then infamous, then a black
knight, then an evil lord.
To see your alignment, you can try "score" or even look on the "who" list.
Note that it is beneficial to stick with a given alignment, since you get
more of a bonus for killing monsters whose alignment is very different from
yours. So if you are very good, you get more experience for evil monsters.
Or, of course, you could stay neutral and therefore be able to go after all
of the monsters, instead of limiting yourself to those of a given alignment.
Various other situations around the mud can affect your alignment, as well
as be affected by it. Some quests require you to good or evil, at some point.
Some guilds require you to keep a certain alignment all the time.
See also: guilds, quests, score, who.