Difference between 2 Ohms and 4 Ohms?

The ohms refers to the impedance of the speakers which is the 'AC version of resistance'. The less impedance the easier it is for the amp, so it can produce more power. This also means that the amp gets hotter, and if it was badly designed or installed, could over-heat.

Incidentally some amps don't like being run with speakers of too low a impedance.

Ohms is the impedance i.e. the electrical resistance the speaker presents to the amp.
The easiest explanation is:
imagine connecting a jump lead across the amps speaker outputs. The impedance of the jump lead is zero ohms (a short circuit), so you would get a huge current through the lead but obviously for a very small time and the amp would self destruct or shut down to protect itself.
Now imagine a speaker across the same terminals. It has a very slight electrical resistance to restrict the current flow & prevent the amp from shutting down. This is the impedance of the speaker, and simply put the lower the impedance the more power the speaker will suck from the amp. However the amp has to be able to handle this additional current which is why you get some amps that are specifie as 2 ohm stable or even 1 ohm stable. i.e. they have been designed to produce more power if the speaker is low impedance.
All amps will run at 4 ohms which is the standard for car audio, but some can run at 2 ohms or even less. Subs are available in all sorts of configurations but you need to check that:
1. The amp is stable at the desired impedance
2. The amp is still stable at 2ohms when bridged
If the spec only lists power values per channel at 2ohms and doesn't list a power when bridged at 2 ohms then you can't use it bridged at 2 ohms.

andy is spot on
the only thing i can add is
some amps specify 2 ohms bridged
some amps specify 2 ohms stereo (but are not capable of bridging)
make sure the amplifier you are thinking of buying is capable of doing whatever you wish

It will but with some you have a switch you need to flick for "low impedance"