Paint comes in a variety of sheens, what you are thinking of painting will dictate the sheen that should be used.
Flat: Provides the best coverage for flawed and imperfect walls and ceilings. It has the highest percentage of pigment in it and is “light absorbing”. Flat paint is harder to clean which makes it a good choice for low traffic areas such as ceilings and adult bedrooms.
Eggshell: This is essentially a flat sheen with a bit more “clean ability”. This is another good choice for areas that don’t see a lot of traffic.
Satin: This has a bit more sheen than eggshell and is easier to clean. This is a good choice for family rooms, foyers, hallways and kid’s rooms. It can be a bit tricky to touch up because it will show brush marks.
Semi-gloss: This is a good choice for areas that see a lot of moisture or grease. It is easy to clean and works well in bathrooms and kitchens, and on chair rails and trim.
High-gloss: Is the most durable of all the sheens, but will also high light flaws. It usually takes several coats to achieve good coverage. It works well on doors, trim and moldings.
Some people have a personal preference, just make sure you understand how durable and cleanable it is for the area you are thinking of painting. FYI we would never suggest painting the exterior of your home in glossy sheens. Besides eggshell or satin on trim, a shiny house looks odd.