Since its creation in 1850, the National Bank is the central bank of the country. However, it is not a public institution, but an autonomous plc. The Belgian State possesses half of the shares, the rest is owned by private shareholders. Since 1999 the National Bank is part of the Eurosystem, composed of all central banks of the euro area as well as the European Central Bank (ECB). Therefore it is responsible for monetary policy and for price stability.
Other activities include lending money to the banks and other financial institutions, banking supervision, collecting annual accounts of Belgian companies and collecting data on credits to companies and indivduals, and managing databases containing economic statistics.
If you want to find out more, visit the NBB's website.