Noobie Question : What is the difference between the booster packs?

Hi,

I've just looked at some videos and thought this game may be fun so ordered a few of the pre-built decks but I have a question I can't seem to find a clear answer to.

What is the difference between the different named booster packs please? Kaladesh vs Eldritch Moon vs Oath of the Gatewatch vs Aether Revolt? And so on.

Thank you so much if you can help.