If you live in that person's household and have access to the vehicle, you need to be listed on that person's policy as a driver. If you reside outside that person's household and are going to drive someone else's vehicle on a frequent basis, that person should list you on that policy as a driver. In either of these scenarios you have regular access to the car and thus the insurer will want to assess your risk as a driver so that they have proper coverage on you and all the other regular drivers.
People who reside outside the household of the car owner may be covered if they use the vehicle only occasionally without being added as a driver. So if you are borrowing a friend's car for a one time use or only every once in a while than likely their current policy will cover you as a permissive driver. It is best however to have the car owner to read over the terms of their car insurance policy and check with his or her current company to find out their rule for this and to verify you would be covered.
Remember, in the future, you can come here to get an online insurance quote and possibly purchase your policy immediately.