Sheen levels are subjective to a degree as different paint manufacturers have different interpretations of how much reflectivity a matt, satin and gloss paint should have. In general, matt paints produce a finish that has no or very little sheen. These paints are usually used on walls and ceilings and are less likely to show any surface imperfections.
Another way to describe satin is eggshell. In other words, the sheen level is about the same as the surface of an egg when 1 coat has been applied. 2 coats will very slightly increase the sheen level.
Gloss finishes have a higher reflectivity than satin or satin-gloss paints. Some paints and varnishes are even labelled as super-gloss. Typically, you will be able to see a good reflection of objects in a gloss paint. Gloss paints are more commonly used for home fixtures such as skirting boards, architraves, doors and so on.