Getting into AOE2

Hey,
I wanted to give a try to AOE 2 since I am getting bored from playing C&C Generals Zero Hour for about 10 years and someone recommended me this game. I got a bit overwhelmed when I searched it, found out there are 3 versions of the game - original, HD and definitive editions, and that they somewhat differ from each other.
What's the difference between those 3? Where plays the majority of the player base? What should I buy and where from (If Steam is OK)?

Thank you in advance! :)