The UNHCR statistics webpage offers apart from data also detailed information on the definitions used, the sources, and the methodologies applied in collecting data on refugees, asylum-seekers, returned refugees, internally displaced and stateless persons: http://www.unhcr.org/statistics.html

More details for the case of internally displaced persons are available on the website of the
The Internal Displacement Monitoring Centre (IDMC): http://www.internal-displacement.org/