While strolling around in the methods of the Graphics class, I bounced on the CopyFromScreen method. He, so why not make a screen capture utility!
I know there is lots of this stuff out there. But it just seems like fun to try to make one myself. I made a little Forms application with two forms: a controllerform and a transparant form on which I can draw a selection rectangle using MouseDown, MouseMove and MouseUp event handlers. I will not give the full code here but just the fully annotated method that does the actual copying and saving. Enjoy!

private void SaveCapture(string Fname)  
    //Fname is the name of the file where you want to save your screencapture
    //you can obtain a filename from a save file dialog
    //construct a rectangle from the topleft and bottomright points of your screenselection
    //if you want to capture the whole screen use Screen.GetBounds
    Rectangle ImageBounds = new Rectangle(_TopLeftPt.X, _TopLeftPt.Y,
                 _BottomRightPt.X - _TopLeftPt.X, _BottomRightPt.Y - _TopLeftPt.Y);
    //now make a bitmap the size of this rectangle
    Bitmap bmap = new Bitmap(ImageBounds.Width, ImageBounds.Height);

    //Create a Graphics object from this bitmap
    Graphics G = Graphics.FromImage(bmap);
    //this is the thing that does it!
    // It copies the colorbits from the screen to our graphics object, wich is a bitmap
    //_TopLeftPt is where our screenselection begins (the source)
    //Point.Empty is Point(0,0) which is the topleft of our bitmap (the destination)
    G.CopyFromScreen(_TopLeftPt, Point.Empty, ImageBounds.Size);

    //save our bitmap in bitmap format
    //this can be changed to jpeg and other formats if you want to
    bmap.Save(Fname, ImageFormat.Bmp);

    //no longer need for these things

It is much better to use using blocks for your Bitmap and Graphics objects.

using (Bitmap bmap = new Bitmap(ImageBounds.Width, ImageBounds.Height))
    using (Graphics G = Graphics.FromImage(bmap))
        G.CopyFromScreen(_TopLeftPt, Point.Empty, ImageBounds.Size);

        bmap.Save(Fname, ImageFormat.Bmp);

That way, if an exception occurs, the objects will be properly disposed of.

The article starter has earned a lot of community kudos, and such articles offer a bounty for quality replies.