Watercolour is a type of paint that is used with water, traditionally applied in washes. The paints themselves are made from powder pigments that are mixed with a water-soluble binding materials such as gum arabic.
Washes are very diluted watery paint layers which give paintings a transparent or light look, often using layers to build up colour. Typically watercolours are used in washes and the white of the paper is used for highlights.
Watercolour paints can come in blocks or tubes (as above). There isn’t one format for watercolour that is better, blocks are perhaps more practical if you are painting outside. Natural bristled brushes are best for watercolour because they tend to be softer, sable is well known to be the most popular choice for watercolour painting. Synthetic stable brushes are now available and these are a good, slightly less expensive alternative to sable.
Like any type of painting having a variety of different brushes will be useful for your watercolour painting. I recommend having at least five different brushes, see my post on the The 4 Most Useful Brushes for more advice.
You will need a big palette with ‘well’ areas where you can have watery washes. Small palettes are more difficult with watercolours if you using them in a very watery way, you will need space on the palette. Plastic palettes are fine for watercolours unlike acrylic paints which stick and dry onto the plastic.
You will need very absorbent paper to use with watercolours. Watercolour paper is very thick and cotton based which makes it ideal for absorbing water. You can buy watercolour paper in sheets and cut it down if needed or in a sketchpad. Both are fine but it generally tends to be less expensive to buy the paper in a sketchpad.
The quality of the materials is important for watercolour, you need good brushes that come to a point and thick absorbent paper to get good results. Watercolour paper can be quite delicate and the less expensive varieties can’t stand too many washes before buckling or losing their surface texture and starting to become ‘fuzzy’.
Also see my post Which Sketchpad? for information on watercolour sketchpads.