Watercolour paintings are painted on high quality watercolour paper and are ready to be popped in a frame.
Watercolours have a more 'free' feel to them than the acrylic paintings.
Acrylic paintings are painted straight on to a canvas which is ready to hang straight on to a nail on your wall. You can choose to get these framed if you wish, but most people don't.
The final painting is varnished which gives a good protection against dust and becoming grubby. The varnish also intensifies the colours, giving an extra 'pop' to the painting.
The colours of an acrylic painting tend to be richer and more intense than a watercolour painting.