The Brush Tool

The Brush tool is used to draw shapes and pictures in photoshop you can change settings on the brush to affect the size which is how big the brush is. Also you can change the hardness of the brush which is how sharp the brush stroke is a soft brush is where it fades off towards the edges of the brush stroke.
You can also change the opacity of the brush which is how transparent and faint it is also you can adjust the flow of the brush which is how much 'paint is applied to an area every time a brush moves.

In photoshop you have different brush styles which gives the brush stroke distinct looks.

The brush options allow you to change the settings of the brushes to give different style and colour effects to the brushes. an example of this is shown below.