Transmutation spells change the properties of some creature, thing, or condition.
This is the opposition/prohibited school for when you choose Conjuration. Conjuration is also Transmutation's opposition school.
For a list of all spells which are of the school of Transmutation, see: Category:Transmutation spells.
Transmutation is a very good school for helping allies, consisting of spells like Haste, the ability score increasing spells like Bull's Strength, as well as Enlarge Person.
There is a distinct lack of hostile spells in this school, but among them is the great Disintegrate, as well as Crumble, Entangle, Burning Hands and Inferno.
It also contains the summoning spell Mordenkainen's Sword for casters who have Conjuration as a prohibited school.
There are also quite a few Druid-only spells, such as Awaken and Drown.
Generally it is a poor choice for Spell Focus due to its strengths in defensive and ability increasing spells, although Extend can help a great many of the transmutation spells.