Note: LianTze was one of the technical reviewers for Stefan Kottwitz’s second edition of LaTeX Beginner’s Guide, reviewed here.
TikZ is an immensely powerful LaTeX package for creating diagrams and graphics. TikZ has grown immensely popular since its first release in 2005, and there are now many other LaTeX packages that are based on it. This includes dedicated packages for drawing specific kinds of diagrams: tikz-cd, tikz-feynman, chemfig, etc.; as well as classes and packages that use TikZ to enhance various typesetting elements: beamer, tcolorbox, etc. Its underlying system is called PGF (a "portable graphics format" for TeX), so you may also see "pgf" in articles that discuss TikZ.
