Spells are categorized by school of magic, which represent the many different philosophies of magic and areas of study among spellcasters.
For example, the School of Alchemy studies the magic of transmutation, so you'll find that most alchemy spells focus on transforming one material into another.
Selecting spells exclusively from a specific school of magic can help give your spellcaster a unique focus, although you are not restricted to any school when you select spells.