What is Forth?
From the Forth FAQ: Forth is a stack-based, extensible language without type-checking. It is probably best known for its "reverse Polish" (postfix) arithmetic notation, familiar to users of Hewlett-Packard calculators: to add two numbers in Forth, you would type 3 5 + instead of 3+5. The fundamental program unit in Forth is the "word": a named data item, subroutine, or operator. Programming in Forth consists of defining new words in terms of existing ones.
Why and where is Forth used?
Although invented in 1970, Forth became widely known with the advent of personal computers, where its high performance and economy of memory were attractive. These advantages still make Forth popular in embedded microcontroller systems, in locations ranging from the Space Shuttle to the bar-code reader used by your Federal Express driver. Forth's interactive nature streamlines the test and development of new hardware. Incremental development, a fast program-debug cycle, full interactive access to any level of the program, and the ability to work at a high "level of abstraction," all contribute to Forth's reputation for very high programmer productivity. These, plus the flexibility and malleability of the language, are the reasons most cited for choosing Forth for embedded systems. Find more information here.
FCode is a Forth dialect compliant to ANS Forth, that is available in two different forms: source and bytecode. FCode bytecode is the compiled form of FCode source.
Bytecode is small and efficient. And an evaluator (bytecode virtual machine) is almost trivial to implement. For example, putting a 1 to the stack only takes one byte in an FCode bytecode binary. This is especially valuable on combined system or expansion hardware roms where the available space is limited.