Juvia has found a home in two guilds so far in the series of Fairy Tail. Initially Phantom Lord and then finally Fairy Tail. So what are guilds exactly? They can be many things from a workplace, hang out place or even home.
A Guild (ギルド) is seen as a magic organization in the Kingdom of Fiore. Typically there are three types: Legal, Dark and Independent. Legal guilds are approved by the magic council with the intent to take on jobs and help the populace. Doing so generally earns rewards such as jewels, etc. Dark guilds are seen as criminal organizations and are generally persecuted by the magic council. Independent guilds are a bit blurry, a grey area if you will. While not a criminal guild, they are not under the scrutiny of the magic council.
While Guilds do generally imply a mage's one, there are also merchant, mercenary and thief guilds.
The hierarchy of a guild can vary from place to place but generally it goes: Guild Leader, S-Class Mages and then regular mages and members. Who gets to be a S-Class mage varies on the guild as well.
The Phantom Lord Guild (ファントムロード or 幽鬼の支配者) was a legal guild based out of Oak Town in the Kingdom of Fiore. At it's height, Phantom Lord was considered of equal standing to Fairy Tail in terms of members, resources and mage's powers. The guilds boasts a wizard saint (Jose) and several S-Class mages (Element 4 & Gajeel). Aside from powerful mages, the guild building itself had a powerful canon called Jupiter.
It's Guild Leader Jose Porla had a long time rivalry with Makarov of Fairy Tail. The result of their fighting caused Jose to never attend a guild leader meeting again. It also left with Jose with bitter feelings towards the rival guild. In X784, Phantom Lord accepted a request from Jude Heartfilia to obtain his daughter Lucy and bring her back to him. Attacking Fairy Tail, a Guild war was initiated. With Phantom Lord's loss, the guild was disbanded with many members simply wondering off to join other guilds. Gajeel & Juvia would go on to join Fairy Tail with Makarov's approval.
The Fairy Tail Guild (フェアリーテイル or 妖精の尻尾) is a legal guild based out of Magnolia Town in the Kingdom of Fiore. It was founded in X686 by Mavis Vermillion alongside Precht and Warrod. It is said the guild remained rather small until Makarov took over as guild master. Fairy Tail was seen as the biggest and strongest guild in Fiore, boasting 100+ members with a large mead hall. The guild could also boast having several S-Class mages including Erza, Gildarts and Mystogan. Becoming a member of Fairy Tail is a rather simple and informal process. You don't even have to be a mage!
Fairy Tail's status in Fiore didnt change until the events of Tenrou Island unfolded. Many key members, including Makarov vanished for 7 years. During this time membership dwindled and with poor leadership, their building fell into foreclosure. Twilight Ogre moved in and took over as the strongest guild in Magnolia. Fairy Tail was forced to relocate to the outskirts of town in a shabby hut looking building. During this time, Macao became the guild leader. With the return of the core members and victory at the Grand Magic Games, Fairy Tail was rewarded its old guild building back.
Fairy Tail has strong ties to the town of Magnolia. They participate yearly in the Fantasia festival in town. The town itself has redesigned itself for S-Class mage Gildarts. The town's houses actually move and walls come up to form a straight line to the guild! Mechanisms! If the town didnt love Fairy Tail why else would they do that? While Fairy Tail is described as a loyal, rambunctious group of people, most of the core core members are either orphans or have a tragic past in some way. Fairy Tail is known as one big family. As to the name Fairy Tail...i'll leave you these words:
"Do fairies really have tails? Do they even exist... Like them, this place as an eternal mystery... A never ending adventure!" -- Makarov