IR Transistor vs IR diode Reciever
You can think of a phototransistor as a photodiode integrated with a transistor.  The diode does the detection and allows reverse current thru it when illuminated, and the transistor amplifies this current.  In reality is it actually more like one device, but that doesn't really matter to the circuit design.

Both a phototransistor and a photodiode are used to permit some current thru them when illuminated.  The phototransistor has more gain, hence more current for the same illumination.  However, it is usually slower.  This should not be a issue for a line follower robot.  It can matter with a fast communication protocol.

A photodiode can also be operated in "forward" or "photocell" mode.  The diode is usually shorted, and it produces a small current proportional to the light intensity.  This can be convenient in some circumstances.  I've designed photodiode circuits both ways.

For a line following robot, my first reaction is to use a phototransistor.