segmentation fault vs page fault

Segmentation faults occur when the memory is not allowed to be accessed (does not exist, or is forbidden). Most frequently they occur when you dereference a null variable or run off the end of an array. Page faults occur when memory that is mapped but not loaded is accessed. They are not errors, and signal to the operating system that it should load the appropriate page into memory.

These two things are very dissimilar, actually. A segmentation fault means a program tried to access an invalid or illegal memory address: for example, 0, or a value larger than any valid pointer. A page fault is when a pointer tries to access a page of address space that's currently not mapped onto physical memory, so that the MMU needs to grab it off of disk before it can be used. The former is an illegal condition and the program will generally be aborted; the latter is perfectly normal and the program won't even know about it.

"Segmentation" isn't at all related to the old "segmented memory model" used by early x86 processors; it's an earlier use which just refers to a portion or segment of memory.