Guild experience can be earned in any of four ways:

1. Completing quests / daily quests that reward experience
2. Killing any dungeon / raid boss with a guild group/raid
3. Winning a rated battleground with a guild group/raid
4. Earning a guild achievement (these are nice, fat chunks of experience so they feel great when you get em)

A guild group/raid is 4/5, 8/10, 12/15 or 20/25 members of the group/raid being guild members.