Artists’ brushes can be divided into two broad categories: those for oil painting and those for watercolour painting. Acrylics can fit in either depending on whether you are painting with acrylics in the oil manner or the watercolour manner.
When choosing a brush for oil (or acrylic) painting artists are interested in the fibres and the spring. A natural fibre like sable or hog bristle will hold the paint better than a slick synthetic fibre but may shed hairs more readily into the paint and may not be as springy as the synthetic. Solvents in the case of oil painting, may over time, damage delicate hairs like sable. But the softer hairs are very useful for blending. There are blends of natural and synthetic hairs to get the best of both worlds and some new synthetics have micropores so they act like natural fibres.
The handles of oil and acrylic brushes are usually longer than for watercolour as the paintings are often larger and the so the artist stands back further to see more of the painting at once.
When choosing a brush to use for watercolour, artists tend to be concerned with three basic characteristics: the holding capacity, the spring, and the shape. How much water and paint it will hold, so the paint doesn’t run out in the middle of the stroke, is determined by the hairs used. The hairs or fibres used can be natural like sable and squirrel which have a pourous shaft and will hold more pigment or they can be synthetic which tend to be more slippery and springier. The shape of the brush is important as different shapes will make different marks. Round brushes are the most popular brushes as they can be used for the most purposes. There are also flats and short flats (brights) for making square edged marks, filberts (almond shaped, this is a squashed round brush), and specialty brushes like mops for washes, liners (riggers) and spotters for fine marks and fans for blending.
The texture of the brush: springy or soft, and the quality of the point if the brush is a pointed brush are also important. You cannot tell the point quality of a dry brush, you must wet it to see if it holds a point. Brushes are usually sent from the manufacturer with a small coating of gum Arabic to shape the brush to show what it will look like when it is wet. It is best to remove this coating by wetting rather than bending as it could damage the hairs a bit.
Watercolour brushes tend to have shorter handles than oil and acrylic brushes and can also be obtained as pocket (travel) brushes.