SPF stands for sun protection factor and is a gel, spray or lotion that reflects or absorbs UV radiation. Working either physically or chemically.
UV raditation is made up of UVA and UVB photons. UVB is responsible for sunburn as it is shorter in wavelength so does not penetrate the skin deeply. UVA is longer in wavelength therefore can radiate to the deeper layers of the skin, promoting the formation of carcinogenic free radicals. Both can intiate skin cancers and premature aging of the skin. So looking for a sunscreen that protects you fully can be a difficult task.
The actually SPF of a sunscreen tells you how much of the UVB rays will be blocked out. An SPF of 15 will block 93% whilst doubling that at SPF 30 will take you up to around 97% protection. However that does not mean you will automatically be more protecred using a higher SPF, in most cases people who use a higher SPF will be more negligent after application, laying out in the sun for longer and not reapplying after 2 hours.
You will know if a sunscrern has UVA protection because it will contain any one of these ingredients; ecamule, arobenzone, titanium oxide or zinc oxide.
La Roche Posay Anthelios SX, £22
contains ecamule -UVA protection.
Neutrogena Helioplex £10 contains arobenzone.
So look for mulit/broad spectrum products that will protect you from UVA and UVB photons. Also never forget to reapply your SPF after 2 hours, wear a broad rimmed hat, seek shade, apply sunscreen 15-20 minutes before going out in the sun, apply under makeup and wear clothing with SPF protection.
Take care in the sun. Hope this helped!