When you fork(), what you do is split the process into two. Thanks to copy-on-write, the two processes share most of their memory and don't take a lot of resources. Initially one copy is the child and the other is the parent, and the only thing different is one bit saying which is parent and child. (Differences tend to accumulate fairly quickly after that.)

A zombie is a process that has called exit, but whose parent process has not looked at it to reap its status.

Preventing zombies is a question of having parents pay attention to the signals from their children. (Or killing the parents, which causes ancestor processes to take responsibility, eventually terminating in the almighty init.)

Anything you want to do with priorities, you have to do. For instance using setpriority.