Why does it look like a snail?
@ on Keyboards
A book called Managing Web Usage in the Workplace tells of examining pictures of old typewriters and finding that it was relatively common for the @ symbol to be included on the keyboard starting around 1880.
@ for E-mail Addresses
Ray Tomlinson first used the “at” symbol to format an e-mail address using ARPAnet in 1971 for a message he sent to himself from one computer to another to test the system, and amusingly, he's repeatedly been quoted as saying he doesn't remember what message said—it was just some forgettable test message—because he didn't think it was a big deal at the time.
@ on Twitter
More recently, if you use Twitter, you know that you indicate a reply to someone by prefacing his or her name with the @ symbol, but it wasn't that way in the early days of Twitter. Users started putting @ before someone's name to indicate that it was a reply, and the people at Twitter noticed and wrote it into the system so that when you hit the reply link, it automatically inserts the @ symbol. Lately, it’s been showing up more as a general symbol to indicate a response. For example, people use it in the comments section to indicate that they’re responding to someone who posted earlier.
Next: How Do You Pronounce @?