Summer is a blessed time, the time to put away those bulky sweaters and enjoy the Sun. But there's one problem: you're so pale that mimes are hailing you as their new leader. You definitely need a little bit of color, but you're also worried about going outside with all of UV rays raining down on you.
So, what's the difference between SPF and UV index?
Providing a definition of "tanning" isn't as stupid as it seems. A tan occurs when the skin absorbs ultraviolet radiation (commonly called "UV rays"). As a response, the skin produces a substance known as melanin, which darkens the skin's outer layers. While many believe that a tan makes us appear healthier, a tan is actually a sign that the skin has been damaged. It's similar to the beginning stages of a burn.
"UV index" is just a fancy term given to the way scientists measure how much ultraviolet radiation is hitting the earth's surface at a given point in the day. UV is an invisible light that is always present, though to a  variable degree. There are actually two types of UV rays: UVA and UVB. They're both damaging, so you'll want to make sure that your tanning products specify that they guard against both.
The actual index is measured from 0 to 10+, so if you're considering sunbathing, it's a good idea to flip on the weather channel or check out a map and see the day's UV index. The higher the number of the index, the greater the amount of exposure you will have to ultraviolet radiation. So you MUST be careful  on days with a high UV index. If you stay in the sun too long, you will burn and peel and then you will be beached like a whale cause it will hurt to move.
SPF stands for Sun Protection Factor. All sunscreens are given a number between 4 and 30 so you'll have to determine which one is best for you. Most dermatologists recommend using a sunscreen with a SPF of at least 15. However, the level of protection a person needs is usually based upon his/her tanning history. For example, people with pale complexions  should shoot for a SPF of 30, while people with darker complexions may need a SPF of as little as 4. To be safe, its always best to ask a pharmacist which SPF would work best for you. And since most drugstores have pharmacists, you can get this info for free. Good thing too, since you'll probably need extra dough to buy all the beer that you'll likely find on sale in the aisle next to the sunblock.
While most people lump sunscreen and sunblock together, they are actually very different products intended for different use.
Sunscreen is a cream or lotion that is SPF rated. It reacts with the skin to create an invisible barrier against the sun. The strength of the barrier is determined by the SPF number. The lower the number the lesser the protection. Most sunscreens should be applied 20 minutes before you go outside so that it has time to work. Using a sunscreen will not prevent you from tanning but it will lower the risk of getting burned.
Sunblock doesn't have a SPF because it protects the body from all UV rays. Usually, sunblock is a thick cream that should contain zinc oxide (this is the white gunk that you sometimes see people put on their noses - it's funny looking, but it's strong). Okay here's where it gets tricky. Some sunscreens with SPF numbers of 15 or higher are erroneously refereed to as sunblocks. If you want a sunblock, don't buy a package with a SPF listed because a true sunblock won't need it.