SPF stands for 'sun protection factor'. Basically, the SPF in sunscreen protects your skin from cancer causing UV rays by multiplying your natural defense against the rays. Sounds confusing, but it's not. Here's how it works - If you burn easily and can generally only last 10 minutes in the sun, a sunscreen with SPF 15 will in theory protect you for 15 times more than your natural protection time, so in this case you will protected against UV rays for 150 minutes. That is however a theoretical time - it doesn't take into account sweating, swimming, or wearing clothes - or any other factor that might rub the sunscreen off or impede it's benefits.
Another thing to note is that once you've used your protection time (the theoretical 150 minutes), another dose of sunscreen will not add more protection during that day. In this case if you've been outside in the sun for your 'protected' time minutes you are best to avoid sun for at least the remainder of the day to avoid getting burnt.
On the subject of burning, there are 2 types of UV (ultraviolet) rays that can cause cancer: UVA & UVB.
UVA are sun rays that cause our skin to tan and wrinkle and over time will produce age posts and skin sagging. Although not as powerful as the UVB rays, it can penetrate more deeply into the skin. UVB are shorter, more powerful ultraviolet radiations from sunlight. UVB is responsible for sunburn - It affects the outer layer of skin (epidermis) and is a major contributor to development of skin cancer. The best sunscreens protect from both UVA & UVB rays.
Different parts of the world have variable burn times depending on how strong the ozone layer is in that party of the world. New Zealand for example has a 'hole' over it and burn times can be very fast.
When it comes to SPF in sunscreen it is recommended you use the highest protecting sunscreen product whenever you are outside - even in winter or when it is a cloudy day.
http://www.spfinsunscreen.com/ brings you everything you could ever want to know about SPF in suncreen, sunblock or suncream.
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