Explain M18 Battery Differences

Can someone outline the basics between the current M18 5.0 6.0 and 8.0? I recall reading that the 6 and 8 might be a lot different from the 5 in terms of inner workings? Sorry LOL I am new here and know nothing. Basically shopping for the best deal at HD right now that includes all 3 tools I want and best combo of these 3 batteries.

I'm buying the string trimmer, hedge trimmer, and blower. Will be adding some M18 FUEL drill/driver stuff later, and maybe the sawzall. But just landscape stuff now.