What is a core dump file in linux? What all information does it provide?
It's basically the process address space in use (from the mm_struct structure which contains all the virtual memory areas), and any other supporting information*a, at the time it crashed.
For example, let's say you try to dereference a NULL pointer and receive a SEGV signal, causing you to exit. As part of that process, the operating system tries to write your information to a file for later post-mortem analysis.
You can load the core file into a debugger along with the executable file (for symbols and other debugging information, for example) and poke around to try and discover what caused the problem.
*a: in kernel version 2.6.38, fs/exec.c/do_coredump() is the one responsible for core dumps and you can see that it's passed the signal number, exit code and registers. It in turn passes the signal number and registers to a binary-format-specific (ELF, a.out, etc) dumper.
The ELF dumper is fs/binfmt_elf.c/elf_core_dump() and you can see that it outputs non-memory-based information, like thread details, in fs/binfmt_elf.c/fill_note_info(), then returns to output the process space.