Could anyone give me a link to an article on NFA theory.

Please note that nfa (nondeterministic finite state automata) theory is a very important and big subject in computer science. Most of the research and the info on the web is computer-language related (parsing actually). I guess you are looking for AI related stuff though... or are you not?

General automata theory links (knowing the basis never hurts):
- Googling for nondeterministic finite automata
- or searching for finite automata
- wikipedia