They are made from pigment, water and an acrylic binder, which forms a hard, clear film as the water evaporates. It is this transparent film, reflecting light from the pigment inside it, that gives acrylic colour its brilliance.
Acrylics dry very quickly, avoiding a tedious delay before painting further layers.
The film is more flexible than that of other media and is unlikely to crack. Acrylics are resistant to water once dry, which means they can be overpainted without disturbing the previous colour. This means that colour cannot be dissolved with a damp brush as it can with watercolour. To thin the colour, simply add water, or a Winsor and Newton acrylic medium. Acrylics become darker in tone as they dry, rather than lighter as with watercolour, so remember to allow for this effect when mixing your colours.