If you're new to MMO's, you may get a little confused reading this forum, since technically mentioning a Guild can be a bit ambiguous because in ESO there are actually two types of guild that can be referenced in ESO.
One type of guild are those that are storyline guilds. While questing/playing through the storyline of the game, there is a Mage's guild, Fighter guild, Undaunted, (and hopefully Thieves later on). These guilds are merely game content that also provide some guild specific skills your character can utilize, as well provide quests and rewards for completing them.
The other type, which is most typically referenced, are Player Made guilds. Guilds are basically groups of players who've come together to quest/adventure/dungeon-delve together, and work in concert to help each other out. This enables players to go dungeon-delving with people they can trust to fill their needed roles, and trust that they're not going to loot chests and take all the good loot (especially loot that more benefits another player in the group). The game provides tools for guilds, like a guild bank for guild members to deposit unneeded items that other guildmates might be able to use. Their own guild chat channel to communicate with the rest of the guild. Rosters and ranks to organize leadership within the guild. Many players make it a point to find one of these guilds versus having to joing a PuG (pick up group) where there's no telling how reliable, trustworthy, or competent, the players you get grouped with will be.