POP stands for Post Office Protocol . When you configure new E-mail software it will often ask for the name of your server running POP3. Headers are created by the computer to help the message get to where it is going. Headers often contain lots of strange and cryptic stuff. Just ignore it.
Some people like to have a big elaborate signature at the end of their messages.
They might want to include their job title, homepage or who knows what. To avoid having to type all this stuff every time they send a message, many people create Signature files. A Signature file is just a regular text file that contains all the before mentioned stuff. It can be automatically pasted to the end of messages to save time. Many E- mail and Newsgroup readers support signature files.