If C is a "portable assembler", then Forth is a "non-portable assembler" (many people roll their own, which resulted in a proliferation of non-compabile versions).

Neat little language, small enough to pick it up in one afternoon, versatile for tiny to small projects, in particular embedded systems and control software, but nothing you should use for anything that involves medium to large size or more than a single developer.

Languages similar to Forth are often used as intermediate languages in portable compilers, in particular ones written for educational purposes, because it's easy to generate code for stack machines (no register allocation needed).