To copy the entire desktop, press Command-Shift-3. The screen shot will be saved to a PNG file that is placed on the desktop.
Command-Shift-4 will provide a cross-hair that allows you to click and drag an area that is saved to a PNG file that is placed on the desktop.
To capture a specific application window, press Command-Shift-4, then press the Spacebar. The cursor will change to a camera, and you can move it around the screen. As you move the cursor over an application window, the window will be highlighted. Click to save to a PNG on the desktop.
Add the Control key to save to the clipboard instead of to a PNG file.
I'm still new to the Mac and don't know as much about it as I do Windows and DOS but my Mac just runs better than any Windows machine. Watch Microsoft's commercial and read the article that refutes Microsoft's implications. My Mac boots from a cold start faster than Windows boots from a wake-up.