What does a treasury department do in a bank?

The primary functions of a treasury department at a bank involve asset/liability management. A substantial amount of time is invested by the department in forecasting net interest income (NII) and measuring the bank's interest rate risk (IRR) or sensitivity to changes in prevailing interest rates. The statistics generated by the department are typically fed to the bank's Asset and Liability Committee (ALCO), the group which is responsible for establishing guidelines for risk taking and balance sheet funding.
The treasury department generally performs other related functions, such as managing the bank's reserve and risk capital requirements, funding the bank's balance sheet through a number of creative strategies (this is typically done in conjunction with the bank's corporate investments unit), and managing the institution's insurance requirements - property and casualty, directors and officers, and BOLL (Bank Owned Life Insurance).