I have a multi-track recorder and I want to record stuff from my rig. I have a Crate XT15R amp (cheap, obsolete, but nice built-in head) and it has a jack on it that has a speaker icon above it. Do I plug a cable from there to the recorder? Is that the idea? Or will that blow up the recorder?

Also, could someone recommend a site for learning these basic things?

Also also, does anyone know where I can get a instruction manual or user guide for my Crate XT15R amp? My Google searches are turning up nothing.
well it won't blow up the record (shouldn't at least, may be wrong), but the best way to get your amps sound is to mic it up. get a microphone, plug the mic into the recorder, and put the mic near the centre of the amp's speaker.
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If thats an external speaker output it will quickly fry your recorder and could damage the amp. Mic it.