Where can I find "Complete NOOB guide to API"?

Hi, i'm a complete noob when it comes to API. i keep seeing everywhere about this 'API' things, i see them on most trading exchange. what is trade api? how to use the code/ command line? what's it for? how to use it to trade? what programs do i need? how do i trade in "automatic mode" or use trading trading bot? (i read some thread about trading bots, but all i see is the command line and codes, and i have no idea how to make it work...hahaha i'm so noob) how do i get real time data update from trading exchange site? etc etc...

I apologise if this question has been asked a thousand times. i tried to search "api", "api tutorial", etc etc... all i find is some advance stuffs that i don't understand. i can't find any threads that explains what is api and how to use it to trade.

if someone is kind enough to point me the right direction i'll be very much appreciated. thank you.