There are two kinds of twins: monozygotic and dizygotic.
Monozygotic (MZ) twins are conceived when a single egg fertilized by a single sperm splits into two after fertilization. Such twins have nearly 100% of their genes in common. Both of them invariably have the same blood type and are of the same sex.

Dizygotic (DZ) twins are conceived when two eggs are fertilized by two different sperms. They can simply be thought of as siblings who are born at the same time. They share averagely 50% of their genes, and their blood types and sex can be either same or different.