Microtransactions are 'small' purchases made in a game (or via some kind of store that allows you to buy stuff to be used inside of a game).
DLC is any additional downloadable content that is not included with the game (so something like a day 1 patch wouldn't be considered DLC, I'd say).
All microtransations are DLC, but not all DLC are microtransactions, generally (before someone comes along with some kind of physical microtransaction or something I guess)
I personally just view microtransations as anything that isn't 'playable content'. So buying a mount from an in-game store would be a microtransaction, while buying an expansion wouldn't be. Map packs kind of blur the line in this instance, because one could argue that they're essentially 'world cosmetics', but its a hard and fast rule and not something I'd try to enforce as a law, ya know?