The general rule of thumb is emulsion on the walls and ceiling, eggshell/satin/gloss on woodwork and radiators. There is radiator paint which is slower in yellowing too, but the stuff stinks.
The paints are made for each job and it's been like that for years. Eggshell are more hard wearing and suitable for woodwork because you're more likely to knock into these parts, going through doors, hoovering near skirting boards etc.
Then you have emulsions for walls and ceilings. There's matt paint (which looks the best to me) but should be used wisely on a whole, not used in a room with kids, high traffic... Also it's not wipeable as a rule and marks easily so not so good in kitchens either. I learned this the hard way as I'd never used flat matt before and it sopped up any grease, and when I wiped over it trying to remove the grease, the paint came off the wall.
Then there's all sorts of various other finishes, a silk finish is ok to wipe on a whole, and kitchen paints which can be wiped without worry at all.
I would advise not painting your radiator with emulsion. It can be done of course, but personally I do not like the finish, looks 'studenty', plus it's not as hard wearing again.
I wonder if it's worth you investing in a good, general DIY book to help you start to learn these very basic things? They can be hugely informative and a real help.