Really not much. I used to make lots of pickles and you can just use regular cukes. You can plant gherkins for the little sweet pickles but if you pick the regular ones when they are small, you get basically the same thing. The only caveat is that if you plant the "burpless" kind they have less acid and don't pickle as well. I always grew straight eight. They are great for both pickling and eating, although they don't get long and stay skinny like the burpless kind.
If you grow your own, just pick them small. However, if you have to buy them, you have to be careful of what you buy (don't get burpless or English cukes) and you may have trouble buying the others the size you need as they let them get lots bigger for salad cucumbers.