Here we explore what types of magic exist in the world of Fairy Tail. We will also visit Water Magic and what kind of edge that gives Juvia. For the specifics on her attacks and abilities see here. If you are not familiar with Fairy Tail, its advised to read about the different types of magic and its origin before diving in further.
Magic has been around in Earthland for centuries and is derived from The One Magic (一なる魔法). The exact origin and history of it very vague, with characters having various theories on it. Some even speculate it is related to Zeref in some shape or form. Magic, the very existence of it, is believed to be the embodiment of one's spirit. When mages form connections with nature on a spiritual level, magic is given physical form. Mages can't use their magic 24/7 though, there are limits. Often called Magic Power (魔力), it seen as a "container" of sorts inside a mage's body. The bigger your container, the more magic power you have.
Particles (Eternano) fill this container and are also found in the atmosphere in Earthland. If one's container should become empty and they still continue to use magic it could have detrimental effects on the user such as death. If they rest, their container is refilled over time from the particles in the atmosphere. Areas of heavy Eternano can have bad effects as well on people. After the time skip, it is revealed through Ultear that there is a portion in every mage's container that goes unused. Through special magic though, it can be tapped into, giving an "upgrade" of sorts to one's Magic Power. This untapped section is called Second Origin, and it something Juvia gets access to before the Grand Magic Games.
As for magic usage, there are two types: caster and holder. Caster Magic (キャスターマジック) is the type that Juvia uses and is related to the body itself. Such as flames, thunder or water coming from the body. The person is able to manifest it on their own. This is very different from Holder Magic (ホルダーのマジック) which needs an object or external source to put their magic through in order to manifest something. Some examples of Caster Mages are: Juvia, Dragon Slayers, Makarov and Jellal. Some examples of Holder Mages are: Cana, Erza, Bisca and Lucy.
Water magic, the type Juvia uses, can be used by either caster or holder mages. Currently in the series, Juvia is the only mage (excluding celestial spirits) that exclusively uses water. The uses for water are vast and as demonstrated by Juvia, can have its properties changed. Depending on its uses it can drown, burn, cut, or sweep foes away. Water can also be used for entrapment and for making pockets of air. Water is definitely one of the more useful and pliable elements in the Fairy Tail universe. Curiously, Ice magic is completely different. Despite making water hot or cold, nowhere in water magic is it mentioned the ability to turn water to ice.