TBT is an ETF that is comprised of investments that roughly correspond to twice the inverse of the daily performance of the Lehman Brothers 20+ Year U.S. Treasury Index. Consequently, TBT will rise in price when long-term bond yields increase and decrease in price when long term bond yields decrease. As with any levered ETF, the issue of time decay is present with TBT, although time decay in treasury ETF's seems to not be as big a problem as in other sectors.
Link to treasury yields: finance.yahoo.com/indices
Link to Yahoo TBT page: finance.yahoo.com/q