It isn't really low or high it is the type of hormones used. In normal contraceptive pills, you get a dose of man made Estrogen and maybe also man made progesterone. This is very effective and mimics pregnancy in effect preventing ovulation and stopped real pregnancy.
Low dose contraceptive pills are really man made progesterone only. They are also known as the mini pill. this pill is much safer and older women can take it also. However, not all P.o.p as they are known, prevent ovulation, they merely thicken the mucus and stop the sperm entering the uterus. So pregnancy can still occur, but they are still pretty effective too - about 89%
Your health adviser will help you decide which is best for you.