Mac OS X has a great feature that allows the users to Force Quit an application which is stuck, and is not responding. This enabled the user to launch the app again and comes pretty handy when troubleshooting apps. This is similar to (CTRL + ALT + DELETE) on Windows machines which helps the user to end the process forcefully. However, compared the MAC the Windows thing is slow, and poor whereas in MAC it enforces the action immediately. There are quiet a few ways to do this and in this guide i will be listing the two most common ones, use the one that best fits your needs and usability.
Method 1: Force Quit using the Apple Menu
Assuming that the app is open, all you have to do is click on the Apple icon on the top left and choose Force Quit.
A window will pop-up listing all the running apps. Click on the App you wish to Force Quit and choose Force Quit.
Method 2: Force Quit from Activity Monitor
Browse to Activity Monitor from Finder -> Applications and open it. Search for Activity Monitor, Click and Open it. Locate the Process and click the X on the top left to Force Quit an app. Activity Monitor, can also provide a lot more information such as the performance statistics, cpu and memory consumption by Apps etc.