The language used for programming FriCAS is called Spad. Currently most detailed and up to date documentation is in FriCAS book at http://fricas.github.io/book.pdf There are changes compared to earlier versions and older documentation, some mentioned at FriCASModifications.

Spad is an imperative language with functional freatures. It provides:

• normal control structures (but no goto)
• convenient "parallel" iteration
• functions as first class values
• function overloading on both argument types and return type
• parametrised types (which allows type-safe generic programming)
• modular structure for large scale programming: categories, domains and packages

Currently Spad does not support object oriented programming, however generic programming features present in Spad in may cases give similar effect.

For more details see A very brief introduction to programming in SPAD. AxiomCompiler gives some historical context.