Transfiguration is the art of changing something into another thing. Animate to inanimate, inanimate to animate, or something from nothing. There are a few rules on what can be conjured, for instance, it is impossible to create love, sentient life, food, knowledge, and money.
Classes start with changing simple things. Inanimate objects to other inanimate objects. Then inanimate objects to small simple animate objects. Animate to other animate objects is considered one of the hardest parts to learn. Finally, conjuring is taught, and it takes a witch or wizard very talented at transfiguration to be able to easily conjure much more than a small object.