As mentioned above, electronic mail relies on a set of standards to work. They provide the "language" used to transfer the messages. If your postman has a letter written in ancient greek he probably won't be able to read it, so it's fairly likely that the letter won't get delivered. The same applies to electronic mail: if the computer doesn't understand the language used by another system it won't deliver the message. This can be the cause of a lot of problems.
You shouldn't need to know anything about these standards, but in case you're interested...
The standard responsible for sending email is called SMTP (the Simple Mail Transfer Protocol). Pretty much everyone uses this. There are alternatives, but you don't see them very much any more.
While email collection usually works as described above, there are multiple standards, all doing pretty much the same thing in different ways (each one has its advantages and disadvantages). The two major standards are POP3 (the Post Office Protocol version 3) and IMAP (the Internet Message Access Protocol). POP3 is the older, simpler and more widely used standard, while IMAP is a newer, but much more feature-rich standard that has been gradually gaining popularity.