Brush Tool

Brush Tool is a primary drawing tool; on the left toolbar.
hotkey: B
Content drawn with Brush Tool first goes onto Spree layer.
More on Spree here
Both Brush and Eraser tools use Bay interface, through which multiple features like sizing, brush selection and other options are available.
More on Brush Bays here
Quick Menu allows for a quick and visual way to change the size of a brush.
Hold Q key, while hovering cursor above the canvas, and move cursor to the right to increase size, and to the left to decrease.
Blending by default is done by holding Z key and drawing with brush. Also brushes may have blending ON by default in brush preset, then holding the key will toggle such brush to draw instead of blend.
More on Brush Library here
Curve Assist is a Brush and Eraser tool option (sync'd for both); it helps with drawing rounder curves.
Higher the level (0 - 7), the more prominent assistance effect will be applied to your brush / eraser strokes.
note: very slow brush work may have minimal assistance even at the highest level, to accomodate detailing.

More Brush / Eraser features are available through Brush Bay interface. More on Brush Bays here