The Hydrometeorological Prediction Center (or HPC) is one of 9 service centers under the umbrella of the National Centers For Environmental Prediction (NCEP) which is part of the National Weather Service (NWS), which is part of the National Oceanic and Atmospheric Administration (NOAA). Tired yet?
The center is currently housed in the "World Weather Building" in Camp Springs, Maryland. A brand new facility is under construction on the University of Maryland campus in College Park. Completion date is unknown as of the time of this writing.
The HPC is primarily responsible for housing the super computers that run and produce the major global weather forecast models (this would be considered the "numerical weather prediction" side of the house).
The HPC also produces several of the more "operational" forecast and guidance products, such as the Quantitative Precipitation Forecast (QPF) (a forecast of surface rainfall amounts),
as well as medium and long range (i.e,. 3 to 7 day) forecast maps
as well as excessive snow and/or ice forecasts, etc. The HPC also takes over for the National Hurricane Center when that office ceases to issue advisories on a tropical system that has made landfall and loses tropical characteristics.
In a nutshell, I guess you could say the HPC is the support center for all of the local NWS forecast offices, as well as the private sector and media interests that are focused on weather information, forecasting & warning operations. All of the various weather observations and other data from around the nation and globe flows through the center, is processed by both supercomputers and human forecasters and is output in various forms to the end users.
You can visit the official HPC Website here.