A segfault, or segmentation fault is a type of memory access error that occurs when attempting to access an area in memory that the process does not have access to. It is usually caused by incorrect pointer use or incorrect memory addressing.
It is raised by hardware that has memory protection, and it will notify the operating system directly. Usually, the operating system will then pass a signal to the offending process.
The common error you will see is SIGSEGV, which is the error code in linux for SIGnal SEGmentation Violation.