I am Baffled By The Word EGO, are you?

I have heard the word ego used many different ways and have read many different definitions of word ego. As a result, I am baffled by the word ego. What is ego really and where does it live? Is ego a word someone made up to describe a condition and if they did, what is the condition? Is ego a bad thing and if it is,why is it bad? Any thoughts you have on the word ego would be appreciated by me.