Photodiodes and Phototransistors

[KerryF]

I am ordering some samples online.  Theres some phototransistors and photodiodes available on there.  I am wondering, what are photodiodes used for, phototransistors used for, and I already know what LDRs are used for.  Are they of any use?

[Paul Perry (Frostwave)]

They are used wherever you want to know how much light is falling on the diode or transistor. Typically to tell when an object blocks a beam.
The idea of a photodiode is, the light shines on the diode surface & generates a current. I have a photodiode in my SpaceBeam optical theremin, to detect the reflected IR beam from the unit.
A phototransistor just amplifies the photodiode built into it. (the base current is generated by the light).
LDRs are more suitable for FX, usually.

[KerryF]

So can a Photodiode be used like an LDR, but instead of having light change the value, its more like a beam?  I can order some samples for free so I was wondering if they would be of use to me.

Thanks.