The word "latex" originally referred to the use of rubber in one form or another as the resin, or solid, in paint. The solvent or thinner, called the "vehicle," was water. Today, many paints are made with water as the thinner but with resins that are not latex, and the industry is leaning toward such terms as "water-thinned" or "water-reducible." If the paints are called latex at all, the term often used is "acrylic latex" because they contain a plastic resin made of acrylics or polyvinyls rather than rubber.
In addition to the speed of drying, new opacity (the ability to completely cover one color with another), and washability of acrylic latex paints, the greatest advantage of water-thinned paints is you can clean up with water. The higher expense -- as well as the potential fire hazard -- of volatile thinners and brush cleaners is gone. If you wash the brush or roller immediately after the painting session is over, it comes clean in just a few minutes.
Latex paint works well on surfaces previously painted with latex or flat oilbase paints. It can even be used on unprimed drywall or unpainted masonry. However, latex usually does not adhere well to high-gloss finishes and, even though it can be used on wallpaper, there is a risk that the water in the paint may cause the paper to peel away from the wall. Because of its water content, latex will cause bare steel to rust and will raise the grain on raw wood.