A guild is a collection of players that have banded together to quest and socialise together. Towards end game it is usually necessary to be involved in raids, astral adverntures and more complicated quests, for this grouping is needed. Guilds take out the search for players, as there will often be guildmates in similar situations.
Some guilds are primarily set up for socialising, as you will sometimes find language or nationality specific guilds, or ones who are more casual than the top guilds.
Joining one can make your Sarnautian life a whole lot easier.