lithium ion is the term used for the umbrella of lithium batteries usually. you've got it right about the packages, usually when someone is talking about a lithium ion they are talking about a battery in a solid metal or plastic case. The lipo's are in polymer sacks. The other difference used to be that li-ion had 3.6v per cell nominal vs 3.7v nominal for the lipo's. New li-ions though usually run at 3.7v. From my understanding the reason why we use lipo in RC is because it saves weight, and also helps with heat issues because the heat wouldn't have a case to flow through, jsut a thin layer of plastic.