Tikz (a self-referential acronym for Tikz Ist Kein Zeichnenprogram)
It's an excellent, and extremely versatile drawing program.
I highly recommend it.
Here's an extensive manual: https://pgf-tikz.github.io/pgf/pgfmanual.pdf
This manual might be frightening at first sight, but it in fact starts very gently, with a long list of worked examples (starting on page 30).
There's also some humorous examples of things not to do when designing a graphic (check out the stuff around page 99).
