What is After Effects and How do I use it?
Adobe After Effects is a industry standard tool used for visual effects and motion graphics. Or, at the very least that's what it's supposed to be used for.
Learning After Effects is a bit of a learning curve, depending on what you want to do.
For example, YTPMVs/音MAD is a type of video where audio is manipulated to make a song, and while After Effects cant make the audio, it is an extremely popular program for the visual side of things.
While I'll try my best in the future to give you information on Shader Graphs, Masking, Layer Switching, Motion Tracking, 3D and Effects. There are people who are way smarter than me who could probably explain it a lot better than I ever could. Here are some people who you should check out for more information, programs that go outside of After Effects. Like Reaper, Cinema4D, Blender, Vegas Pro and more.
MMaker - Teaches the basics in After Effects and has a really good tutorial series on his YouTube.
Blender Guru - You can probably guess what this guy does.
REAPER Mania - Makes really good REAPER tutorials, except he explains things.
SonduckFilm - Motion Graphics Tutorials
YouTube Search - Find any tutorial on any topic (wOW TeChNoLIGY)