Q: How to take screenshots in MacBook?

            Command-Shift-4-Space   - Capture Window under cursor, then Click

            Command-Shift-3               - Capture entire screen

        If you want to capture a menu or pop-up, then use

            Command-Shift-4-Space

        NOTE:  Preview can be used to annotate a Screen Shot with Circles, Arrows,

                    and Text to describe interesting points in the Screen Shot.

        Screen Shot to Clipboard:

            Command-Control-Shift-4               - Drag to select copy region

            Command-Control-Shift-4-Space   - Capture Window under cursor, then Click

            Command-Control-Shift-3               - Capture entire screen

            Now paste the clipboard into whatever a document that accepts graphics such as Mail, TextEdit, Preview, etc...

        If you want a menu or pop-up captured  in the Clipboard, then use

            Command-Control-Shift-4-Space