The most important thing to understand is your liability in case of accident. There are five main charges you will incur if the car gets crinkled:
1) Physical damage/theft. You will be liable for pay for repairs to the rental car, itself. This can be mitigated by your personal insurance, if you have comprehensive/collision insurance. Many credit cards provide secondary coverage (i.e. pays for deductible), but some do provide primary coverage (pays for car up to a set amount before insurance).
2) Liability (medical). If someone gets injured in the accident, and it is your fault (or the other person is uninsured), you will need liability coverage. This is also usually handled by your personal auto policy, but some corporate accounts can add/replace liability coverage.
3) Loss of Use Fees. These are fees the rental agency charges due to having an asset unable to make them money (car out of service). Sometimes credit cards may cover this, but usually required proof from agency they actually needed the car, which the agency doesn't provide. these can run ($1,000+).
4) Administrative fees. Rental agencies charge an admin fee to file all the paperwork and get the car repaired. This is $200-$1,000 usually.
5) Diminutions of Value fee. If the car sustains damage that makes it unable to be resold (i.e. after certain structural damage or flooding, car is considered salvage), the rental agency will charge a fee to recoup the loss of the asset's value.
Try to see what your personal auto policy will cover. If you don't have an auto policy (some people don't have cars), then you can purchase one if you like. Of course, the rental agency will sell you coverage for each of these items, if you prefer.
Good luck and have fun!