Acrylic paint is fast-drying paint containing pigment suspended in an acrylic polymer emulsion. Acrylic paints can be diluted with water, but become water-resistant when dry. Depending on how much the paint is diluted (with water), the finished acrylic painting can resemble a watercolor or an oil painting.
Acrylics are sometimes used in place of watercolors because acrylics dry closer to the desired color (slightly darker, usually) while watercolors dry lighter (and often unpredictably, especially for beginning artists).
Acrylics are often used as an alternative to oil paints because acrylics dry much faster. Oil paints, which consist of pigment suspended in an oil (usually linseed, or other natural oil) base, can take a very long time to dry. Acrylic paints can achieve an oil-paint-like effect, and do so in much less time. Though applied to look like oil paints, acrylics are somewhat limited due to the superior color range of oil paints, and the fact that acrylic paints dry to a shiny, smooth (some say 'cartoonish') effect—not surprising since acrylic paints are, basically, plastic. Accordingly, acrylic paint can be removed with turpentine, mineral spirits (also known as white spirits), ammonia, or rubbing alcohol.
Acrylic painters modify the appearance, hardness, flexibility, texture, and other characteristics of the paint surface using acrylic mediums. Watercolor and oil painters also use mediums, but the range of acrylic mediums is much greater. Acrylics have the ability to bond to many different surfaces, and mediums can be used to adjust their binding characteristics. Mediums can change the sheen from gloss to matte, or can add iridescence or texture to the surface. They can also be used to build thick layers of paint: gel and molding paste mediums are sometimes used to create paintings with relief features that are literally sculptural.
Acrylic paintings, ideally, should be treated as if they're as different from oil paintings as are watercolors: they are their own artform. There are techniques which are available only to acrylic painters, as well as restrictions unique to acrylic painting. Therefore, judging an acrylic painting as though it were an oil painting (or a watercolor) is not always appropriate.