I’ve also been using the GIMP and Inkscape for a few years now, as well as Paint.NET more recently for light-weight editing, and they are all excellent software! They all have fairly good help files and online documentation, so if you haven’t found those yet they are a great reference for how to use specific tools & features.
The simplest definition of alpha channels are how layers have transparency – it allows a layer to have “nothingness” in addition to/instead of a color. (So in addition to having your red, green, and blue color channels you have another one for NOT color.) There’s a lot you can do with them to blend layers once you get the hang of it, but don’t sweat it at the begninning. 🙂
The way I generally think about (& use) raster vs. vectors is painting vs. erector set. Raster images you have a thing with dimensions that you do stuff on and what you get depends somewhat on the order and you can’t always undo, exactly. Vector is more like a rubber erector set, or pins and rubber bands & flexible sticks on a corkboard that you can move around and tweak until you like it.
I have examples of stuff I’ve done in both but most of that is stored on a server I don’t have hooked up right now… my avatar was done in Inkscape, though, as was this T-shirt design for a friend (attached).
Ray-tracing does use vectors but mostly in the rendering math, not usually too much in the scene creation. That’s a whole ‘nother can of worms, but also a lot of fun. 🙂
They call me.... thread killer.