The classic fork bomb, meaning a process that duplicates itself exponentially, will use up two resources: the CPU and the process table. The CPU, because each of these processes need timeslices to run, and the process table because suddenly there is an explosion of processes.
"Resource hogging", can refer to any computing resource. It doesn't have to be a fork bomb. For example, if you have a
dd running and it uses up all I/O bandwidth to your disks and your machine becomes unbearably slow due to high load, you could technically say that
dd is being a resource hog.
Same can be said for any program that tries to take up RAM (Example: "Chrome is a resource hog, it eats RAM for breakfast") or disk space, or any limited resource on your computer.
Now, finally: causing your system to crash is really just the very rare case. Your system will get slow, and in the case of memory starvation the OOM Killer (Out of memory killer) may cause mayhem. An outright crash, however, is really the exception.