[VoiceOps] What does an ALG actually do?

If you look for "SBC" or "B2BUA" as synonyms for "SIP ALG", I expect you'll find lots of thing people think they're good for. On Wednesday, February 27, 2013, John Levine wrote:

I realize that an ALG is a hack in a router that is supposed to allow SIP packets to go through a NAT router. I also realize that for modern SIP equipment, ALG usually causes more problems than it solves, and that it's described in RFCs 2663, 3424, and others.

What I can't find anywhere is what a SIP ALG actually does to the packets. Is that written down anywhere, or is it just network folklore?

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