It was established by resolution 827 of the UN Security Council, which was passed on 25 May 1993. It has jurisdiction over certain types of crime commited on the territory of the former Yugoslavia since 1991: grave breaches of the 1949 [Geneva Conventions]?, violations of the laws or customs of war, genocide and [crime against humanity]?. It can try only individuals, not organizations, governments etc. The maximum sentence it can impose is life imprisonment. Various countries have signed agreements with the UN to carry out custodial sentences.
See also:
References
The tribunal's website is at http://www.un.org/icty/.