What is Paint Tool SAI?

I've heard that lost of people use this tool to draw. I downloaded a free version for mac because I wanted to try digital art, and I used this really old bamboo tablet with it. None of the drawings were very nice to look at… XD The tablet I used didn't have a screen (it was pretty old), so it was hard for me to see what I was doing- that was probably part of the issue. Right now I use Pen & Ink on my iPad, which works pretty well, but I would like it more if it had more brushes and tools.
Anyways- I really want to try this program out, so if I could get a few details on what the program is like, what kind of tablet would be best with it that would be so great. Also, if you know of another good digital art program I could use that is better than Paint Tool SAI or is easier to use, that would be awesome.

Thank you in advance!

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