What has to be compatible?

[nmt6789]

I dont know much about computers so I have to ask this question.... When building a new computer what has to be compatible such as mobo, CPU, hard drive, ram???

 

[macdude425]

Mobo and CPU need to be compatible.
HD/CD drive interfaces need to be compatible with mobo.
RAM needs to be compatible with mobo.
Case and PSU need to be compatible with mobo.