In select mode you can drag points around the screen. Any objects dependent on the points will adjust accordingly.

This will add points to your sketch.

If you click any line or circle, it will divide that object into equal pieces. You can do this as many times as you like.

Those new points can be used to make any normal objects, but can't be dragged.

This will add lines to your sketch.

If you click existing points, they will be used. Otherwise, new points will be created for you, and used.

This will add circles to your sketch. The first point assigned will be used as the center, and the second will determine the radius.

If you click existing points, they will be used. Otherwise, new points will be added for you.

Clicking things will remove them from the sketch.

If you remove a point, then any object which depends on it will be removed, too.

If you remove a line, then its midpoint will be removed, too.