The Internet is fast and inexpensive because the major protocols adopt a packet switching strategy. The early Internet was the first packet-switched network. The program usually works on some kind of data chunk. Data and programs are stored in files . The files have to be in the main memory in order for the protocol to happen.
Email is for sending and receiving messages to other people.
User agents let you compose, read, reply to, and forward messages. The user agents takes your message and puts it into MIME format. Mail servers handle shipping and receiving messages, resending and producing error messages, and notifying the user agent when mail has arrived. The mail server probably runs on your ISP's computer. File Transfer is for shipping files from one host to another.