What is This OSR Game Anyway?

Tell me about that OSR game you've been harping on about.

The Setting

1) How is it like D&D?

2) How is it not like D&D?

Character Creation and Advancement

3) How is it like D&D?

4) How is it not like D&D?

Playing the Game

5) How is it like D&D?

6) How is it not like D&D?


Note 1: Your point of reference for D&D may be different to mine, but I think there's enough of an assumed core there that you don't need to tell me whether you love OD&D or 5e. The answers will probably make that clear.

Note 2: I will get around to answering this for Into the Odd soon.