If only we knew…
As outlined at http://www.helpguide.org/mental/pdf/Colic.pdf, colic is a general term used when a baby cries for more than 3 hours a day for more than 3 days a week. We are not entirely sure what causes the crying though there are many theories about it. About 1 in 5 babies have it. The important thing is that the baby is otherwise thriving. If he or she is not putting on weight as expected then you should take them to see your doctor as there may be another reason for the crying.
One of my junior colleagues has put together a very comprehensive article on colic for doctors who use my other website which supports their continuing medical education. The language for that particular article is not too medical and you might find some of the background theories interesting.
See also http://www.northlondonpaediatrician.co.uk/?p=84 on whether it is normal for babies to cry.
Babies grow out of colic but not soon enough for most parents. Make sure you have lots of support and get a break from the crying every now and again. You can always put the crying baby in his/her cot and walk away for a few minutes to calm yourself down.