I don't know all of the stuff, but I do know most of them.
1. Rectangle select: you select different areas, and you can't draw out of the selected area. When you are on it of you click out of the skin in the empty area it goes away.
2. Move selected Pixels: Well that what it does it moves selected pixels.
3. Lasso selection: don't know
4. Move selected pixels: does what it says
5. Eclipse Selection: It selects stuff in a circle.
6. Zoom: left click to zoom in, and right click to zoom out.
7. Magic Wand: selects area with similar colors.
8. Pan: When your zoomed in you can move around with it.
9. Paint Bucket: fills a area with a specific color.
10. Gradient: well, that doesn't really help with skin, but it makes part of the picture darker and slowly lightens.
11. Paint Brush: basically a bad pencil.
12. Eraser: erases things.
13. Pencil: lets you draw on the skin.
14. Color picker: one of my favorites. when you click on it your color that you draw with becomes the color you clicked. Very helpful.
15. Clone Stamp: when you click a part of the skin on it you get another stamp, and when you draw in the empty area it becomes the area the other stamp is on. very helpful for 3D detail.
16. Recolor: if you have it on a light color, and you just tap it it shads and highlights the area you tapped.
17. Text: lets you write.
18. Line/curve: makes a line or curve.
19- 22: makes shapes such as rectangle, rounded rectangle, eclipse, and freedom shape.
That's it hope it helped! If it did please diamond, or fave. both would be great too! subscribe to stay updated with my stuff.