Alignment is a necessary and unfortunate side-effect
of your actions.  It most clearly comes about as the
result of killing people, but that is not always the
only modifier to alignment.  Some quests, a few
guilds, and other unique situations are determined
or influenced by your alignment.  Players who kill
evil beings become more good, players who kill good
beings become more evil.  To consistently keep the
alignment of good or evil, one must also kill
consistently.  Neutral beings have a range of
alignment somewhere in between. 

The following guilds have alignment restrictions:
Cultist (evil)
Fallen (evil)
Necromancer (not good)
Monk (neutral)
Paladin (good)
Templar (good)

If you do not have the appropriate alignment,
you will not be able to join the guild, advance
in levels in it, or train skills/study spells
in it.  Note that the more powerful the monster,
the more good or evil it is.