What should I say when some one ask me what is Appium

Hi, I have being attending Appium sessions, prior to that , my idea about Appium is that , its a tool which is used for executing scrips on any device
After attending the initial sessions, I am not fully conviences with statement "Appium is a tool". My understanding is that , Appium is actually a server. Appium also provides API in form of Appium java-client.jar [AndroidWebDrive, ...etc] , through which we execute the scripts on mobile or any device [by communicating to device browser or app]. 

But for that, we need Android SDK and othre stuff to make our device recognise and let execute scripts on device.

Now , for the question, what is needed for mobile automation? should i say, Android SDK/Studio or Appium.

I need clarity on this. Please correct me if i am wrong.