a freewheel is a cog that rotates around another peice with little bearing/bb lookin things on the inside. These will last you 3-6 months probably. They are on average $15 a peice. You can get them in sizes 13-17? and they thread onto your hub.
a cassette is a hub with a built in free wheel of the sort, but it uses springs and pawls ussually to engauge. These are rebuildable and will last people probably 6months to a year without much problems. you can run 12-17? gearing with these.
a driver is the peice that the cassette hub uses for the cog. stock hubs have a threaded driver so you can interchange different sizes. You can get a one peice driver that will allow smaller gearing 8-11t so you can run the micro gearing stuff.
a freecoaster (you forgot these) are considerably different and use little pad deals that scrap on the inside of the hub to engauge (basically) so that when you pedal forward it engauges but when you back pedal the hub will coast freeeeely while going fakie. you use a driver on freecoaster hubs too, im pretty sure. KHE hubs are a little different, I think it more applies to nanaki odyssey and poverty type hubs.
ok so now that i understand that, im planning on slowly replacing parts on my bike and something that gave me a bit of a problem was gearing. if my freewheel now has 17t and my chainwheel has 40t, how would changing the sizes affect my bike? most sizes that i saw were around 14-16 and 2?-39t. how would a 16/39 setup feel compared to my original setup?