Hi Stella,
I agree - all those paint types can get confusing.
Unless you want your walls to scream SHINE, definitely stay away from semi-gloss and gloss paints.
Either a flat or satin paint is your best bet for walls. While flat is less likely to show any imperfections in your walls, the satin is much easier to clean. If your room will get a lot of people use - especially from kids, I would go for satin. I have 5 kids and have painted every room in my home for the past 20 years with satin paint and just love it. The sheen is very low -and, it's worth its weight in gold to me for the easy maintenance factor.
I always paint my ceilings with a flat paint. It has virtually no sheen and is low-traffic.
I only use semi-gloss on doors and trim.
So basically, when I'm painting a room I buy 3 types:
satin - walls
flat - ceiling
semi-gloss - doors & trim
Also, when you paint the room - do the ceiling first.
If you were going to paint bathroom or kitchen cabinetry you could use satin, semi-gloss or gloss - depending on how shiny you wanted it to look. Cabinets get a lot of fingerprints, so they definitely need a paint that is easy to wipe down.
Hope that helps!
Donna
