That's actually a hard question, since a guild can be different things depending on the guild.
Some are very strict with the rules, some are just a loose collection of friends.
At it's core, a guild is a group that plays the game together for a common purpose.
What the common purpose is, that's what makes each guild different.
A linkshell is kind of like a guild. For the most part, in FFXI, a linkshell was a chat group. A guild is more organized, since you can't switch guilds that easily (in most cases), you tend to get attached more to the people and get closer.
Omusa - 66 Tauren Hunter - Illidan
Diafp - 17 Blood Elf Mage - Illidan
Seoman - 70 Dwarf Warrior - Anetheron
Seomusa - 26 Draenai Shaman - Anetheron
"Americans just aren't gullible enough to believe that they came from a fish," John Morris, president of the Institute for Creation Research in San Diego