Your signature vs your mark

Is there a difference between your 'signature' and your 'mark'? One of the comments on this post on Bruce Schneier's blog claims there is:

This might be out of date in these days of 100% literacy (yes, that is sarcasm) but it's not actually your "signature", it's your "mark". This is why serious contracts require witnesses.

If there is a difference, then what exactly is it?

Traditionally, if a party to a contract were unable to sign their name, they would literally make their mark (and often get a witness to sign that they had made the mark themselves).

The quote is from a comment by "bob" at May 4, 2011 3:40 AM on the previous-day's blog entry by Schneier. Most of the 55 comments use the term signature to refer to a person's name, written in cursive by that person. (A few comments also refer to digital signatures, which are arrangements of digital data rather than instances of handwriting.) "bob" is commenting on an 11-minutes-earlier comment by "Danny Moules":

"I sign everything with a poorly drawn picture of a dog. I've only once ever been stopped for this, by a grocery clerk."

That is, "bob" asserts that the dog picture is a mark, an identifying symbol, rather than being a signature in the name-written-in-cursive sense.