“Watercolours” is a term that refers to all techniques that dilute paint with water.   Acrylic, gouache and tempera are the other, opaque, types of watercolours. Aquarelle has as its most important characteristic that it is transparent. It is made of vegetable, animal and mineral pigments that are grinded with water and thickened with Arabic gum. For paint in tubes, glycerine is added for fluidity.
Traditionally, when creating an aquarelle, no white paint is used: paper is white and where the image is white, you leave out the paint. The white of the paper is the only white used in an aquarelle. Aquarelle paint is transparent, lighter shades are achieved by thinning the paint with more water.  Aquarelles often have a fresh and spontaneous look: the transparency does not allow for endless “repairs” by applying many layers…
Aquarelle has the reputation of being very difficult and it is: it has to be “right the first time”. You have to work fast  and do some careful planning of the composition before starting. Decisions have to be made, about the composition, where the light comes from and, thus, which parts should be left white.
When things have gone wrong in an aquarelle, my way to go is to tear it up and start all over again, remember, it is just a piece of paper… That said, smaller mistakes can often be repaired.  I sometimes tear it up but keep the good part, as inspiration. 
Aquarelle has a poetry that other forms of graphical expression cannot touch! It is very direct and fast and therefore ideal for “catching moments”. Oil and other heavy  media have to be built up and take time to dry.  They are much less suited for working fast.
Another great advantage of aquarelle is that it doesn’t require much. You can easily take it with you to go paint outside:  A jar of water, a piece of paper, a tiny bit of paint, a few brushes and you can go ahead.  It doesn’t smell, it dries fast and isn’t messy.
To be successful with aquarelle, you need to be able to mix colours and learn to balance the amount of water and the amount of pigment on your brush. You can achieve beautiful results with a big brush and loads of water, working wet in wet. Being economical with water brings more control. But whatever amount of water, the material “leads its own life” and you can use that. Washes (layers of the thin, watery paint) are influenced by gravity, wet parts “bleed” into each other and the paint dries very fast in the sun or very slowly when it is humid. Big wet drops dry up with a dark rim around them, watery paint pushes pigment away and colours get lighter when they dry up and there’s much more… To be successful with aquarelle you need lots of practice and also patience and spontaneity.
Use the effects when sometimes the material (paint and/or paper) seems to have a mind of its own – it can lead to stunningly beautiful images…