Sigdashes are a (de facto) way of specifying where your mail ends and your signature starts. They're pretty cool, because smart mailers and newsreaders can do funky things when they notice sigdashes.
For example, many mail clients will strip off old signatures when replying to mails. This is a Good Thing, because, hey, just one signature per mail ya?
Many mail clients, like mutt, can display signatures in a different colour or font.
So, what /are/ sigdashes?
The character sequence "dash dash space" on a line by themselves are collectively known as sigdashes. It looks something like this (without the quotes): "-- "
Configuring your mail client to use sigdashes:
Pine: Setup | Config - Composer Preferences | Enable Sigdashes - Reply Preferences | Strip From sigdashes in reply
Mutt: (sigdashes on by default) in .muttrc, add set sig_dashes=yes
unless it's set to "no" in /etc/Muttrc or ~/.muttrc, you do not need to do anything.
Thunderbird (via TagZilla): In the TagZilla | Formatting screen, set Tagline Prefix to (without quotes) "\n-- \n"
Thunderbird (no TagZilla) / Evolution / Web based mail: Include the sigdashes line as the first line of your signature file/text.
Kmail / Outlook Express: (No idea)
Go forth and spread the good news.