In SQL Server:

I don't know where to find any documentation on what the "@" symbol means and why it is used. I've done different kinds of advanced google searches for this (e.g. "inurl:forum "@.symbol") and I get squat. I've seen it used plenty of times, but nobody explains it...

Can anybody explain what this is and what it does and how to use it? Or maybe a link to a site that explains it?