Very little. I suppose, technically speaking, ovulation is a single even that happens when the egg is released from the ovary. Then, you have about 5 days that follow after ovulation when you are most fertile. If you have a period cycle of about 28 days, ovulation is usually around day 14. Making days 14 - 20 your "fertile days." this is important if you are trying to conceive.
It's timing. Fertile days are those days when you can potentially get pregnant. Ovulation is the release of a mature egg that can be fertilized. Since sperm can survive up to 6 days and an egg must be fertilized within 24 hours, your fertile days are roughly the week leading up to and a day past ovulation.