The Mac screenshot tool is built into keyboard shortcuts that can be accessed using Shift+Command. Below are the 3 different ways a screenshot can be taken on a Mac.

Entire screen: press Shift-Command-3

Part of the Screen: press Shift-Command-4

A Specific Window: Press Shift-Command-4. Then press the Spacebar. Bring your cursor over the open window you want to take a screenshot of and click it.