Interior Paints: Most paint jobs that are done indoors are done with latex-based paints. You can get latex paints in a ton of different colors and it is easy to clean up by just using water. If you have rooms that don't get used a lot the best paint for you would be flat or low-gloss. For the rooms that are used mor often such as your children's bedrooms, kitchens, laundry rooms, and pantries you should look at paint with a glossier finish. Paints with a glossier finish can handle more wear and tear as well as more cleaning.
Paint Usage: If you are only doing one coat, one gallon of paint will cover 400 square feet. It will change a little bit based on the quality of the surface, the color of paint, and how thick you make the coats. If you hire a professional, they will use the right amount of paint needed, where a home owner will usually use too much paint. Your gallon will last longer if you do two thin coats opposed to one thick coat, and your walls will look better too.
Interior Wall Surfaces: Wall surfaces vary a lot. In most modern houses and remodels you will find the walls are finished with textured drywall's. The textured look will give the effect of bumps or ripples. A lot of modern homes will also use smooth drywall which almost looks like glass. Many older homes have plaster on their walls which looks like putty has been trawled on a lath. In commercial buildings the lath will be metal, on wallboard's it will by gypsum lath, and in pre-1930 homes, the lath will be wooden strips. The finished product will look a lot like the drywall you find in modern homes however the plaster is much more brittle and hard.
Preparing for the Painter: You will need to move anything that could get in the way of where the painter needs to go. You should cover your furniture with drop cloths and move them away from the walls to protect it. You will want to make sure that your baseboards and corners are clean. Usually the painters will take the time to cover the floor and tape anything that needs to be protected.