What was the Renaissance and what caused it?

The Renaissance (from the French word meaning "re-birth") was a cultural movement spanning (depending on which cultural historians you believe) from the 14th to the 17th centuries.

I'm not sure you could say something caused the Renaissance as such, but the "re-birth" which gives the period its name is the re-birth of classical culture, which was rediscovered and re-examined during the period. Artists, writers, sculptors, musicians and architects began to look back to Greek and Roman times, and seek out ideas and inspiration from ancient texts.

The changes in the Renaissance were social and political as well as intellectual, primarily because of an emphasis on secular, rational understanding of the world beginning to displace the previous emphasis on religious or spiritual understanding.

Lastly, it's probably also worth pointing out that some historians question the usefulness of the term 'Renaissance', and ask whether the cultural phenomenon it described ever really occurred.