Acrylics dry incredibly fast, allowing for many layers to be created on surfaces quickly and easily.  Some artists that generally prefer oils sometimes see this see this ability to be a hindrance to blending and mixing colors.  There are, however, acrylic mediums available that will slow drying times making acrylics behave a bit more like oils.  Acrylics dry to a plastic surface that is water-proof.  They can also be thinned and used like watercolor paints.