GDB

GDB is a tool developed for Linux systems with the goal of helping developers identify sources of bugs in their programs. In their own words, from the gnu.org website:

GDB, the GNU Project debugger, allows you to see what is going on `inside’ another program while it executes — or what another program was doing at the moment it crashed.

When reverse engineering a program, the tool is used to review the compiled Assembly code in either the AT&T or Intel flavors to see step-by-step what is happening. Breakpoints are added to stop the program midstream and review data in the memory registers to identify how it is being manipulated. I will cover these steps in more detail below.