1. What it is
A CR2 is a 3-volt lithium cylinder, about 15.6 mm wide and 27 mm long. It holds roughly 850 mAh, so it runs far longer than a coin cell doing the same job.
2. Where you will find it
Instant cameras, film cameras, laser rangefinders and a few car remotes take it. Some cameras take two of them side by side for 6 volts, so check how many the compartment holds.
3. It is not a CR123A
The CR123A is a similar 3-volt lithium cylinder but fatter, at 17 mm wide and 34.5 mm long. It won’t go into a CR2 slot, and a CR2 won’t fill a CR123A holder properly.
4. It is not a rechargeable 16340 either
The 16340 is about the same size as a CR123A, not a CR2: 16.3 mm across and 34 mm long. It is rechargeable and gives 3.6 volts, so it is neither the same size nor the same voltage as a CR2.
5. Keep the spares apart
Keep them in their packaging, away from coins and keys. A CR2 lying loose in a bag can short out against a key and get hot in your pocket.