Cat - TripleSeven, made by Hodgdon (yes, the same folks who make Pyrodex), is a black powder substitute. It's sugar based, so doesn't have the same sulphur stench as real black powder. It produces something like 15% more power per unit of volume, as compared to black powder. Keep in mind this is an approximation, because even different brands of real black powder have different amounts of power per unit of volume.
If you're going to load up with 777, and have been shooting real BP before, you'll have to reduce your loads. As an example, full house loads in my Traditions Hawken are 110grains FFg Goex. That's about as much as one dares put in 'er. To achieve the same amount of "bang" with 777 one would not want to exceed 95 grains of FFg 777. And keep in mind, these are grains by VOLUME, not by weight. So your usual method of using a volume powder measure is just fine for shooting 777, just back the charges off by 15% and you'll be fine.
TripleSeven has some known issues. Firstly, it does appear to require a slightly hotter spark to make it ignite. I switched to #11 Magnum caps and have had no ignition problems, even after leaving the gun loaded for a week of deer hunting. The other known problem with TripleSeven is the "crud ring" - a ring of fouling which appears to build up in the barrel right about where the base of your bullet would sit when loaded. This is no big deal - especially if you are in the habit of swabbing between shots. In the field you'll have not much difficulty loading enough shots to kill a deer or two without swabbing. I find with stout hunting loads that I can get up to 10 shots without really having to worry about swabbing. Obviously for target work I load every shot the same, so swab between every shot. Heck, it only takes an extra second, so why not do it anyway?
So, that's the low-down on TripleSeven as I know it from my experience. I personally love the stuff, and I've dealt with a few whitetail bucks that have learned the hard way not to mess with a "fifty" full of TripleSeven!