The International Criminal Court (ICC) was established in 2002 as a permanent tribunal to prosecute individuals for the most serious crimes of international concern, including genocide, crimes against humanity, war crimes, and the crime of aggression. The court can only prosecute crimes committed on or after July 1, 2002.
The ICC is a court of last resort. It will not act if a case is investigated or prosecuted by a national judicial system unless the national proceedings are not genuine, for example if formal proceedings were undertaken solely to shield a person from criminal responsibility. In addition, the ICC only tries those accused of the gravest crimes.
Try defining "gravest crimes", though, in a world where crimes daily occur too terrible to mention. The United States, controversially, is opposed to the ICC. Current cases before the ICC include, the Lord's Resistance Army's abuse of the Acholi in Northern Uganda, leaders of conscripting child soldiers in the Democratic Republic of Congo, and the situation prevailing in Darfur, Sudan.
Being a relatively new organization, its usefullness is still to be seen, but the potential for the ICC to either be a catalyst for great justice or great harm is a looming unknown.