Magic is the art to control or influence something by using supernatural means, such as spells, charms or rituals. Within the Final Fantasy context, magic plays an extremely important role in both plot and game mechanics.
Magic is a double-edged weapon; it can be used to either save the world or destroy it. The use, or the misuse, of magic can greatly influence the world’s workings or events. One example can be found right in Final Fantasy IV: the destruction of Mist alone (caused by a spell that summons Bombs once Cecil and Kain reach the village) catalyses Cecil’s estrangement from his King and the Reign of Baron and it also completely changes Rydia’s life. Again in Final Fantasy IV the misuse of Meteor, by a weak Tellah, leads to his death.
Stepping outside the world of Final Fantasy IV, another example is Final Fantasy VIII, where the ultimate aim of SeeDs is avoiding Ultimecia’s Time Compression, a sort of very strong time magic that will merge together past, present and future creating a world where she alone can live.
Game-wise, magic is one of the two main ways for attacking an enemy. It is the opposite of physical or melee attacks and, depending on the game, there are two or more types of magic. Common ones are black magic (offensive function, usually inspired by the four traditional elements; earth, wind, water, fire), white magic (defensive function), summon magic (either offensive or defensive function), blue magic (spells learned from enemies), and finally time and space magic. Characters who are able to control magic are usually called mages or summoners.
Being a summoner, Rydia is naturally able to evoke Eidolons but she is also skilled in the black/white magic department. As a child she can use both basic white and black magic, while as a grown up she loses the ability to cast white magic spells. Compared to Palom, the “official” black mage, Rydia learns black magic spells later because, as her class suggests, her focus in on summon magic.
Fitting with the traditional idea of mage, Rydia equips rods, however she can also deal physical attacks using knives and staves, which are substituted by whips when Rydia is an adult. Rods are used to launch magical attacks or to summon Eidolons, whips instead can be used to control stronger Eidolons with whom staves or rods wouldn’t work. Moreover whips, being long, enable Rydia to attack with the same efficacy both near and far enemies. Another advantage of using whips is that they can occasionally paralyse the foe.