Typically, this is a half dozen lines or so that briefly include key information about what you are and what you do. It's important to make this information as compact as possible. You don't want to annoy people with an overly long or flowery message. Some newsgroups and other automated handlers of e-mail will truncate a signature after a certain number of lines. Six lines should be considered the maximum.
Some authorities even advocate four lines.
You should have the ability to switch your signature on or off, depending on the intended recipient. You can generally set your e-mail program to automatically include your signature file on all messages. If you don't want to send a signature you have to switch this off or manually delete the signature. We like to have the signature file switched off as our default condition as it's not always needed for routine or ongoing contacts. There are certainly times when it's appropriate to include the signature.