1- The fastest and default option in Linux is by pressing the "Print Screen" key this key is located in the upper right part of the keyboard. It will capture the total of the screen. After pressing this key the system will ask you where you want to save the screenshot image.
2- Alt + Print Screen (Print Scrn) makes a screenshot of a window, pressing this key combination will capture the window that is active. The system will ask you where you want to save the screenshot image.
3- Alt + Print Screen (Print Scrn) allows you to select the area you want to capture, for this click and drag the pointer across the screen to select the area to print.
4- Application "Gnome Screenshoot": this program included in linux by default allows to perform some more advanced functions such as using a timer, include the pointer or not include it and apply effects.