New to PostGre SQL asking about write-ahead-log (WAL)

Hi,

I am new to PostGre SQL and want to ask some questions. In my database
class, we plan to work on a class project based on WAL. Since we are not
familiar with WAL, we don't know what's a good start point. Could anyone
point me to any documentation mentioning about WAL in PostGre SQL? It will
be very helpful if anyone could point me to the file which includes the
core of WAL implementation. Any advice along this line is very welcome.

Thanks,
Mingzhe