Unlike human health insurance, where you may have a certain list of doctors that you can visit that are covered by your insurance or types of medicine you can take, pet insurance leaves these choices up to you. They cover every licensed vet in the US, so you can take your pet to whomever you please and still get coverage.
Many pet insurance companies do have deductibles as well. This is an amount of money that you are expected to pay before the insurance company will pay anything. For example, if your plan has a deductible of $100, the pet insurance company will not pay the first $100 of coverage. If you have a vet bill of $300, they will pay for $200, but you have to pay for the first $100 yourself. If your bill comes out to be $75, then the pet insurance company will not pay for any of that coverage.
When you sign up, you are typically offered a variety of choices and different plans from the company that you choose. The different plans offer lots of options, such as the type of coverage you want and how much money is covered. Some plans offer just accident and injury, some add on illness, and still others have packages that also include annual vet exams and vaccinations. Some companies offer full coverage of vet visits, while others just pay a percentage of your vet fees. Sometimes they will offer to pay all of your fees, not just a percentage, but there is a maximum payout that they will cover. Some may have deductibles, while others do not. The price that you pay is determined by your company based on a number of factors, including breed and age of pet.