RAR is a archive format used by popular proprietary RAR archiver, created by Eugene Roshal. There are two major versions of format (v1.5-4.0 and RAR v5+).
File format essentially consists of a linear sequence of blocks. Each block has fixed header and custom body (that depends on block type), so it's possible to skip block even if one doesn't know how to process a certain block type.
This page hosts a formal specification of RAR (Roshall ARchiver) archive files using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.