Spin2cpp is a program for converting Spin, BASIC, or C code to binary, PASM (Parallax Propeller 1 assembly), P2ASM (Parallax P2 assembly), C, or C++.
fastspin is a slightly different front end to the same functionality, designed to be compatible with other spin compilers.
The command line options for spin2cpp and fastspin are documented in Spin2cpp.md and Fastspin.md respectively.
Documentation for the various front ends (languages accepted as input) are in doc. The BASIC language is the best documented. The C and Spin documentation just cover extensions and deviations from the standard (C99 in the case of C, and the original Propeller compiler in the case of Spin).