DFHack is a Dwarf Fortress memory access library, distributed with scripts and plugins implementing a wide variety of useful functions and tools.
The full documentation is available online here,
from the README.html page in the DFHack distribution, or as raw text in the ./docs folder.
If you're an end-user, modder, or interested in contributing to DFHack -
go read those docs.
If that's unclear or you need more help, try the Bay12 forums thread or the #dfhack IRC channel on freenode.