QuantMath is a quant analytics library, which contains useful quant-math functions (see the math module). More importantly, it is a framework for allowing a risk or pricing system to interface to quant code, so that a database of instruments can be priced using prices and market data from Bloomberg or Reuters, and risks, P&L attribution and scenarios can be calculated.
The facade module contains the main entry points into QuantMath. For convenience, these methods and the data members they use are published from the root of QuantMath. All of these methods are also available in the C interface to QuantMath, and will be made available in other language-specific interfaces. However, this does not mean that you are restricted to using only these methods when interfacing with QuantMath. For example, you can use the methods in the data and risk models to directly construct market data objects, and the methods in instruments to directly construct products or indices.