Amp Modeling Software. Emulation. Effects Modeling. Fake & Take. Whatever you want to call it. Here is a decent one.

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Also look for Amp modeling VST's. They are plug in effects that process the guitar sound with parameters that allow you to shape it any way you like.

Cool thanks I'll look into it.

"re-amping" is another popular term. Can be done with software or a second amplifier.

Re-amping really isn't done w/ software. That's modeling or emulation.
Re-amping means taking a previously recorded direct guitar (or any signal for that matter), sending it to an amp, and then mic'ing that amp.

One might do this b/c:
1. You don't have an amp available at the time, or don't have the channels to mic it the way you want.

2. You weren't happy w/ the original amp sound/recording.
Solution? Replay the same part through a different amp and/or w/ different mic'ing.
Just tell the guitarist that his amp really DOES sound that good. :wink:

3. To add a different color to the recording.
Duplicating a track doesn't add anything but amplitude. If you change amp or mic (or mic placement), you now have the same part, but different.
This can be used just to thicken things up, or to use a single guitar to make a stereo image (amp A and/or mic A on the left, amp b and/or mic b on the right). Adding a slight delay (-30ms) to one recording can help w/ this purpose.
It is not the same as double-tracking, which means recording different performances of the same part, and relies on the subtle variations from performance to performance. This is often more effective than reamping for the purposes of thickening and stereo imaging.

4. Whatever reason makes sense. When I don't have a channel open to burn on a room mic for the drums (I use a large space), I'll often send a mix of the drums to a speaker in the live space, and put up a room mic.
Not quite the same, but still useful.

How to do it:
Amplifiers want a different level of signal than what most interfaces put out. Amps want Gui/Inst level, interfaces send a mic/line level.
A DI box takes an instrument level and converts it to mic/line level.
If you have a PASSIVE DI, you can run it in reverse. Otherwise, you'll have to get a "re-amping" box, which essentially does the same thing.

I'll leave the "which is better" to someone else.

That said, there is maybe one exception. Some modeling software lets you choose and place a "mic" in front of the simulated amp. Not true re-amping, but does do a similar thing.

Sure, using a computer app to modify the signal in the sense of trying to get it to sound like it's coming from a different amp than the original might not technically be re-amping, but the idea is essentially the same whether you use a computer or an actual amplifier to do it. You're right, the more sophisticated software comes much closer, with choice of pre-amp, speaker configuration, mic type and position and all the knobs and dials on the original amps. But at the end of the day, it's a digital simulation, not an actual amp. I think of it as a spectrum with pure analog re-amping on one end and then a whole range of different levels of software emulation down to really basic and simple software processing. It's sort of like asking 'how many hairs do you need on your chin before you have a beard'? No hairs makes the answer easy. Tons of them makes the answer easy too. And somewhere in the middle is where it gets all weird and rather arbitrary. How close to a real amp does it have to get before we call it re-amping? Unless you want to get all analog purist on me, really it doesn't matter, the important thing is you get the tone you want, whether you use an amp, software, or a combination of both.

Good description of the process and some of the reasons you'd want to do it too!