SPF?

Recently I've been wondering what exactly SPF means and found this simplified answer on http://www.wiki.answers.com/:

SPF stands for sun protection factor. Higher SPF numbers would allow a person to be in the sun longer without burning. SPF 30 means that your skin will not burn until it has been exposed to 30 times the amount of solar energy that would normally cause it to burn. In other words, if you start to burn at 10 mins, applying an SPF 30 would know take 300 mins. to start burning (30x10= 300). However, there are many limitations to this rating (time of day, skin type, uv index, sunblock brand, water, sweat etc...) Also, SPF is just the protection factor against sunburn or UV B radiation and does not have any relation to UV A radiation, which causes long term skin damage.