The question you are asking is so broad and complex that it can't be adequately answered in forum. Postfix is an MTA: mail transfer agent, otherwise known as an SMTP server. It moves mail from one location to another. It works behind the scenes of what most people think of as an email server, which is a POP or IMAP server.
Postifx contains two primary configuration files, main.cf and master.cf. Main.cf is use to set the behaviors of Postfix and is where you specify things like your domain name, your address range, and the restrictions on who can send mail. Master.cf is the file that controls the daemon processes that make up Postfix.
The other files, such as alias maps and transport maps are used to customize the delivery and handling of the mail for one or more domains. Using them one can create a mail system as simple as having mail delivered to local Linux account holders up to a full scale production system hosting multiple virtual domains where authentication is performed via database lookups. These files are used to configure how and Postfix looks to perform these functions. You can also map one mailbox or domain to another one, for example.
A good place to read about these functions is the
Postfix Documentation site.
