Brazing

I've seen it done on my AC lines but what is it and how is it different to soldering?

From what I saw, a hotter torch was involved and (what looked like) solder just plonked on without any flux.

I want to braze someday.

Why aren't all copper water pipes joints brazed?

Unless it's under a slab it doesn't need to be. Brazing rod is much more expensive than 95/5 or lead free solder. Also it's real easy to blow a hole thru 1/2" or 3/4" with oxy-acetylene. Brazing is pretty much overkill for residential plumbing.

Copper will braze without flux using a good solder, usually 6 to 15 % silver, although the 1% will also work. It does take more skill, and a better torch.

As a practical matter, brazing is somewhat deleterious to the joint, because the heat required detempers the copper and the fitting, making them both susceptible to damage and distortion. I have seen the branch lines to tees "fall" off after brazing when the line was subjected to movement.

On the AC lines they normaly pump nitrogen throught the line sets while brazing so there is no flaking inside the lines due to the high heat used...

The flaking could cause premature failure of the compressor...

Did they do that with yours???

Yes they did.

Generally in plumbing brazing isnt very common, but I do it quite often.
I often braze large copper waterlines in residential towers. I use a cutting torch with a special tip, special care must be taken to adjust your tip and when heating your pipe, it takes alot of practice to be good at it, I wouldnt trust many people to do this. Its expensive but once your good at it (quick), it can actually be more cost effective then roll groving and using victolic fittings.
The dangerous part is you can melt the inside of the pipe without knowing it, shrinking the wall thickness and strength, Ive had to repair a building like this in the past. There were holes popping up all over the place just after a few years of use. You really need to get a good feel for it.