"Generation" refers to the blocks of time in which the car remained basically the same. For example, the current accord body style started in 2003, so the 2003-2006 accord is the 7th generation of the accord.

The 1990 to 1993 period is what you are looking for (4th gen). Most parts will be interchangeable for those years. There are minor variations which occur in the middle of a models runs. For example, the 2006 accord has a slight redesign on the rear, and some minor engine improvements, but is basically the same car as 2005.

So if you have to replace a future part on your car, you can look for 1990 to 1993, rather than just 1990. I don't know what minor variations there are within that time period, so I'd search the forums to verify if a particular part is in fact interchangeable. But it usually will be.

I'm sure somebody will correct me if I messed up these numbers!