The doctrine of abrogation (naskh) in Islamic thought is the claim that later verses of the Qur'an can abrogate earlier ones (or, more generally, that later revelation from God can abrogate earlier revelation). It is quite controversial among Muslim theologians (both in the strict form -- whether abrogation in this sense is possible -- and in the more general form -- how far such possibility of abrogation in revelation extends). Jim Benton recently had a good discussion of the Qur'an and the doctrine of abrogation at "100 Camels Times X", which has led me to think about it a bit more. Some resources:
"Niche of Truth" lays out the basic ideas involved in abrogation. I also recommend the discussion of al-Nasikh wa-Mansukh in Chapter 5 of Ahmed von Denffer's Ulum al-Qur'an.
Mansur Ahmad criticizes the doctrine of abrogation at "Muttaqi.org". His criticisms are the most common ones against the doctrine.
The Wikipedia article for Naskh is surprisingly good, at least as a place to find leads for further research.
The dispute is an important one for Islam, because on the one view those who do not know the principles of naskh will misapply revelation; on the other, the reverse is true.