A marshaling point for several loosely connected models in discrete mathematics,
based on free monoids.

Symbol processing systems, automata,
symbolic dynamics for dynamical systems, strings of DNA.
Languages.
Finite state Markov machines.
Syntactic machines.
Systems of strings of symbols.
Artificial chemistry.

The Turing machine
and λ-calculus are classic examples,
but there are many ways of operating on strings of symbols.