This is very important when you are replying to a mail server, you might want to send your reply privately or not at all. If you want to praise someone then by all means do it in public. Everyone likes to hear nice comments about themselves and when it is done publicly, it's even more powerful. This is a small block of text appended to the end of your messages, which usually contains your contact information. Most email programs can automatically add a signature to your messages.
Balance is the key to a good email signature.
Keep your signature short, four to six lines is a handy guideline for maximum signature length. Very Long signatures can be annoying. Poorly written e-mail will be viewed as spam and discarded.