The aggregate data are collected from many continuously reporting fixed stations at the Earth's surface and represent the input of approximately:
6000 temperature stations
7500 precipitation stations
2000 pressure stations
This work is often used as a foundation for reconstructing past global temperatures, such as NASA's GISTEMP. The average temperature record is 60 years long with ~1650 records greater than 100 years and ~220 greater than 150 years (based on GHCN v2 in 2006). The earliest data included in the database was collected in 1697.
References
Peterson, Thomas C. and Russell S. Vose (1997). "An overview of the Global Historical Climatology Network temperature data base". Bulletin of the American Meteorological Society78: 2837–2849. doi:10.1175/1520-0477(1997)078<2837:AOOTGH>2.0.CO;2.