DKS,
Don't be too worried about not understanding it all...sometimes I wish I didnt!!!
To map it out for you so it makes sense.
The Operating System is the "Language" that the computer uses to function. Most people use Windows (e.g Windows 95, 98, XP, 7....or another version) or MacOS. Say you computer speaks "XP" (Italian), those computers who speak 7 or MacOS ("French"...just for the sake of it) can communicate but only with translation.
Installed on the Operating System is a set of programs. These can be word processing ("Word), spread sheets ("Excel"), games, accounting programs, building software...the list goes on. These are the things you actually do stuff with. Like type a document, play a game. Sort of like a drill spins the drill bit...but the drill bit drills the hole. Without one the other doesn't work.
Your Son is using at school a program called "Microsoft Word", this is the most common "word" program...but there are others, one of these is "wordpad"...but "Microsoft word" and "Wordpad" speak different dialects of the same language, so things get lost in translation.....I hope this makes sense. Microsoft word is however much much more advanced than wordpad...and much more important that he learns to use this. Wordpad is like trying to use one drill bit for all materials, while it might work in the end, it will take a long and frustrating time to do it.
For your Son to easily use his files saved at school ate home he can use "Microsoft word"...the same as he uses at school...or you can use something like "Open Office" which will do the same thing but the computers at is School may not be able to "translate" the data back. Microsoft word you *will* have to pay for, while openoffice is free to download.
Anyway I hope this makes some sense to you. Good luck with the learning, I'm jealous in a way.
Pok