You can take a picture of your entire screen, ...
just the focused window, ...
- Screen: PRINT SCREEN
- Focused Window: ALT + PRINT SCREEN
- Selected Area: SHIFT + PRINT SCREEN
These shortcuts will save a file with the screenshot into you Pictures folder. The same combinations with the CTRL key will copy the image to clipboard instead.
You can find these combinations and more in: System Preferences → Keyboard → Shortcuts