What is a port?

I have been running an Apache web server for numerous years and have been using ports as a medium to serve requests. Recently, I began using Unix sockets to serve MySQL requests. The function of Unix sockets I understand as: - the 'socket' is a specific location in memory, represented as a system file with extension '.sock'. When used as a communication medium, a process can write to (or read from) this file to communicate with another process. What, and where, are system ports ? Do files exist in the filesystem that facilitate the same communication purpose as Unix sockets ? If I wish to interface with, say, port 80, and interpret a HTTP request, how could I achieve that ?