Alkyd paint is more common because it is less expensive and tougher. Oil-based paint is good for trim work because trim takes more abuse over time than do walls. Latex paint is easier to work with and dries more quickly, but it isn't quite as durable as oil-based paint. Latex is good for general painting projects such as walls and ceilings. Paint comes in a number of different finishes, from flat to high gloss.
Some paints also contain enamel, an additive that makes the dried surface harder and less porous.
Keep in mind that there is no true economy in buying cheap paint. Always buy the best-quality paint you can afford. And when choosing colors with paint chips, remember that the color on the wall will be more intense than it appears on the chip simply because the surface area is much larger. If you're working on unfinished walls, be sure to apply a coat of primer before you begin painting. Use latex-based primer if you plan to use a latex paint.