The selection tool allows you to select points, lines, line segments, etc. by clicking on them with the left mouse button.
The point tool allows you to make a point anywhere in your sketch simply by clicking the left mouse button.
The compass tool allows you to make a circle by left clicking where you want the circle's center and then expanding the circle to the desired radius, then left clicking again.
The straight edge tools allow you to make lines, rays and line segments.
To make a line, use this tool. Left click once to make one point, then click again to make the line that passes through those two points.
To make a ray, use this tool. Click once to make the starting point of the ray, then click again to make the ray starting at the first point and passing through the second.
To make a line segment, use this tool. Left click where you want the segment to start, then click where you want it to end.