Sorry if it's been posted already; screenshot help?

Seems as though I've accidentally run into the screenshot mechanism, of course I cannot remember WHERE. I read about the "shortcuts" to it but I won't remember that so is there a one click way to do it?

Preview has File > Take Screen Shot.
Utilities > Grab is an App for Screen Shots.
Finder has Command - Shift - 3, and Command - Shift - 4 for a section of the screen.

There's also Command + Shift + 4 then place crosshair over any window and press spacebar to change the crosshair into a camera icon, then left click on window to take screenshot of just the window itself and it's contents.

If you need to abort, just press Esc.