Paintbrush Tool functions like the Pencil Tool, allowing you to click and drag to create a path. The difference is the Paintbrush Tool applies predefined vector art to the paths you create. Keyboard shortcut for this tool is B. A path created with the Paintbrush tool can be reshaped with the Pencil or Direct-selection tool.
Double click the Paintbrush tool to open the tool setting dialogue box. Low fidelity value will produce a path very close to the way you drag the tool. A high setting will produce a smoother path with fewer anchor points that only approximates the path you drag. Check the “Fill new brush strokes” box if you want new paths to be filled with the current fill color.