The GNU debugger
GDB is a source-level debugger for the compiled languages, primarily C and C++, although there is also support for a variety of other languages such as Go and Objective. You should read the notes for the version of GDB you are using to find out the current status of support for the various languages. The project website is http://www.gnu.org/software/gdb and contains a lot of useful information, including the GDB manual.
Out of the box, GDB has a command-line user interface which some people find off-putting although, in reality, it is easy to use with a little practice. If command-line interfaces are not to your liking, there are a lot of front-end user interfaces to GDB and I will describe three of them later.