Excerpt from the wiki on email:
Header fields:
The message header should include at least the following fields:
From: The e-mail address, and optionally the name of the author(s). In many e-mail clients not changeable except through changing account settings.
Also note that the “From:” field does not have to be the real sender of the e-mail message. One reason is that it is very easy to fake the “From:” field and let a message seem to be from any mail address. It is possible to digitally sign e-mail, which is much harder to fake, but such signatures require extra programming and often external programs to verify. Some ISPs do not relay e-mail claiming to come from a domain not hosted by them, but very few (if any) check to make sure that the person or even e-mail address named in the “From:” field is the one associated with the connection. Some ISPs apply e-mail authentication systems to e-mail being sent through their MTA to allow other MTAs to detect forged spam that might appear to come from them.
Sender: Address of the actual sender acting on behalf of the author listed in the From: field (secretary, list manager, etc.).
Details on http://en.wikipedia.org/wiki/Email
For example gmail uses the from/sender fields to send emails from different email adresses than your gmail account (After verification).