A while back I bought a couple of IR LEDs from eBay. They were labeled as “IR Emitter and Receiver” (see picture).
The emitter seems to work just as an ordinary LED, but I am not sure about the “receiver”. Each time I’ve asked anyone about this they say “don’t you mean light sensitive resistor / photo transistor”, but the thing is, when I shine an IR source on the receivers I get a voltage between the terminals, no resistance change.
When I first tried to use them I figured out that putting them “backwards” in series with a resistor gave me values to analogRead. By backwards I mean the long terminal to GND and short to 5V. The value of the resistor changes the sensitivity, the higher the R the more sensitive the circuit.

Does anyone know what it is that I got?


Yes that is the correct way to use an photo diode although I would swap the diode and resistor.
Basically the diode is reverse biased. Then photons of light produce electron hole pairs in the intrinsic region of the junction. These are then seeped into the anode / cathode and look like a little bit of conduction.

Normally this is used with a transistor or op amp to amplify these small changes iin conduction.

Thank you very much for the answer.
I am currently using them in a tachometer, using the reflection of IR on the item which is spinning to measure RPM. However I am in a bit of a problem with this, I can't use the tachometer if there's too much background light. The light causes it to always read as digital 1. If I use a lower value for the resistor I am not sensitive enough to pick up on the reflections of the spinning item. What would be a good way around this problem?

The standard way round this is to use modulated IR, this allows the use of a tuned filter in the amplifier this blocking any "DC" interfering IR from daylight.