Trim and extend tools are highly useful for a wide variety of tasks inside of Illustrator. These 'smart' tools automatically edit artwork based on mouse click points or proximity to other paths.

Trim tool - by clicking
The trim tool automatically deletes the excess portion of an intersecting path or the distance between the click point and the closest endpoint. Try trimming an Illustrator line or set of Illustrator lines that cross an object.

Trim tool - by dragging
Trimming can also be as easy as dragging over a set of intersecting lines. Select the Trim tool and drag the cursor over the artwork to watch the lines instantly trim to the nearest intersection path.

Extend
The Extend tool automatically extends a path to meet another path. This is particularly useful for creating a forced boundary for lines without covering them with fill 'patches.' To try this tool, create a set of lines that approach a curved path. Select the Extend tool. Then click on each endpoint, then click on the curved path to extend the line to meet it.