Quoting the Pysam Github repo description:
Pysam is a Python module for reading and manipulating SAM/BAM/VCF/BCF files. It's a lightweight wrapper of the htslib C-API, the same one that powers samtools, bcftools, and tabix.
I do not know which pybam you are referring to, you'd need to provide a link. It is probably any other project aiming to write software for NGS data manipulation in python.