1. thin
2. ultra-light
3. extra-light
4. light
5. semi (semi-light)
6. book
7. normal
8. regular
9. roman
10. plain
11. medium
12. demi (semi-bold)
13. bold
14. extra (extra-bold)
15. heavy
16. black
17. extra-black
18. ultra (ultra-black)

normal, regular, plain and book - Standard font thickness.

It's clear that the smaller the font, the less levels you can apply, you just can not see the difference.
For example, in the figure below, the Arial font is shown in an enlarged size with a size of 10 points: from the top - normal, from the bottom - bold.

As you can see, the difference is insignificant. The font itself did not become thicker. It was as thick as 1 pixel, so it stayed. It is clear that in this size it is simply impossible to make bold, super-fat ...

With a large font size, the subtleties will be more noticeable. If you are familiar with web programming, you know that in CSS, the font thickness can be set in percentage - from 1% to 100%.

What is the difference between Italic and Oblique?

It's very simple. Italic is italic, that is, a special version of the font, something like printed and written. Oblique - oblique, that is, exactly the same font, only tilted.