GDB is the GNU project debugger.

help gets help.

b sets a breakpoint, i.e. b class::function or b filename:linenum.

run (with optional args) runs the program, you'll probably want to set a breakpoint first.
step into functions
next step over functions

print {expression}

{empty cmdline} repeats previous command.

bt prints a backtrace.

list lists the next few lines of source.

dir {directory} tells gdb where to find the source code in case you're not running in the same direcory that you built from.