Brushes are simple technology.
Paint is collected in the hairs, and applied to the canvas or board by the force of the painter’s hand on the handle.
ANATOMY
The belly and tip of the brush may be viewed as a thick forest of hairs, where liquid paint is held, to be squeezed out by the force of the brush against the canvas or board.

The brush’s capacity for storing paint is determined by the belly; the shape of the tip and overall length of the hairs determine the shape of marks, and springiness of the brush.
TYPES
Disregard the exotic types, such as fan, mop, and cat tongue, until you really know your way around the 3 basic types. You could paint a life’s work with a set of flats and some rounds.

Personal preference applies; I much prefer the solidity and angular marks of a flat brush, while my studio neighbor may like the flowing strokes of a tapered filbert.
NATURAL OR SYNTHETIC
The hairs may be synthetic or natural.
Stay away from the super-cheap hog’s bristle brushes, as these will frustrate you.
Same advice goes for high-end sable or mongoose brushes; they offer value mostly to the accomplished painter, who needs the last touch of springiness.
Your workhorse brushes should be a nice set of synthetics.

Over time, paint works its way up into the brush’s ferrule by capillary action. The paint sits in the ferrule, evades cleaning, and dries. This dry paint puts a force on the hairs, forcing them apart, gradually making the brush ragged and less precise. Stray outer hairs may be snipped with scissors, but at a point, the brush needs to go.
Remember, I urged you to get an affordable workhorse set? Replace ragged brushes, and you won’t get frustrated.
CLEANING
As we’re using water-mixable oils in this series, cleaning is simple.
To keep your brushes healthy for as long as possible, clean them in this way:
Add a drop of dish-washing liquid to the brush. Grip the handle with one hand, the hairs with the other, and gently work the hairs collectively round in figure-eights under the tap, until the brush is no longer emitting paint.