Good for cutting stuff out that's hard to get to with a square. If you let go without reaching the beginning of the line it'll cut an extra straight line to it. It's 2 pixels thick which is annoying and confusing.
~SELUCT

whatever you select you can move around (with the mouse..), duplicate (ctrl+move), spread (shift+move), delete (delete) and copy paste and all that malarky. I guess you know how to copy and paste ya?
You can select the secondary colour to become transparent when moving around, makes things easier sometimes.

~RUBBA
To be honest you shouldnt really need this. I usually have the background colour as my secondary colour so i can delete as i go and see fit. If i want to delete lots i usually use the select tool or paintbrush (although the largest rubber size is bigger by a few pixels than the largest paintbrush, oohlala).

Fills the space you click. Has absolutely no tolerance. Good for filling things in quickly.
~pic culla.

Pick colour. Left click for primary colour, right for secondary. You'll use this lots because the recent MSPaint has lost the save palette and everything things.

Goes 2x, 6x and 8x. Before you pick however the default zoom is somewhere between 2x and 6x. You can also do 10x by clicking the minute area between the 8x bit and the bottom of the box. Right click to return to 1x. Fun stuff.

Your trusty pencil. Makes 1px dots. Quite restricting, and can make alot of mistakes if you're me, and when drawing lines it'll make some 2px thick. Before releasing the click, tap the other clicky button on your mouse and the stuff you drew will dissapear as if it were never there. incase you make mistakes. Can do that with basically any tool but just thought i'd say now.
~Brush
Like pencil but can be bigger. I only really use the circles. Never use it much anyway, perhaps for sketching out and rubbing out.
~AERBRUCH

Is evil. Never touch.

~Techst
Can only be used at 1x zoom. All the usual fonts are ther. Click the thing to make the secondary colour transparent so that the text box doesnt delete a load of stuff. Dont use it in pixel art, but you can use it to write stuff, notes and shx.
~Lyne

THE TRUSTY LINE TOOL. I used to use this very often. Good cus the pointer is pretty presice and you can do pencil stuff and more when you need it. Don't use bigger thicknesses than 1px, cus they suck.
~Kurv

Ew, confusing and useless. Unless you wanna make big horrible car pics.
~Rectumble
Good for making squares and rectangles. You can select it to only do an outline, do an outline of primary and fill of secondary, or just do a fill of primary, in that ordera. Press shift while doing your business to make a regular square.
~Polygon

Was that a pokemon? um anyway I don't use this so it must be useless unless you wanna make horrid big stuff.
~Ellie ps.

*cries over his ex* Ok well this is ALRIGHT at making circles. By ALRIGHT i mean that once you make one for the love of god edit it so it doesn't look horrible and jaggedy. Does same stuff as rectangle tool.
~Rounded rectangle
What's the point of this? Just make a rectangle and curve the points yourself.

 

Hope that helps someone. Fufufufu.

Oh and the polyline is good when you want to conncet distant points while zoomed in.

Oh and move this topic lar-lar.

 

You should write a tutorial for more complex things. The basic interface is very good actually. You can't get much more clear than what MS did here.

 

If you think you have found an area that is difficult for some people then by all means go ahead. :)

*backs away into shadows*

Dithering and AA should be basic, selective outlining and stuff like that advanced, and the lightsource, palette, and lines should be in a 'Fundamentals' section.