Zombie Movie - This is a movie that features the undead. Zombies typically eat brains and move slowly. There are always exceptions, granted.
Outbreak Movie - This movie focuses on the outbreak of some sort of disease. The disease is usually very contagious and causes the infected to revert to a more primitive state where the focus is on survival.
Here are some examples for clarification's sake:
28 Days Later - Outbreak movie
28 Weeks Later - Outbreak movie
I Am Legend - Outbreak movie
Zombieland - Outbreak movie
Shaun of the Dead - Zombie movie
Dawn of the Dead - Outbreak movie
Planet Terror - Outbreak movie
Fido - Zombie movie
Night of the Living Dead - Zombie movie
I hope this cleared things up for you. There are far more "outbreak movies" than there are zombie movies. Please stop mixing them up, it vexes me terribly.