A structured investment vehicle (SIV) is a special purpose vehicle that buys long term bonds and other fixed income securities, funding this with by issuing short or medium term debt such as commercial paper. The securities an SIV buys are often mortgage books or some other from of asset backed security.
SIVs are also called conduits because they create a channel through which the long term debt they invest in can be funded by short term debt. They have also proved to be a conduit through which banks have bought back mortgage debt that they had apparently off-loaded through securitisation — although the banks that buy may not be those that sold, the risk comes back into the banking system.
The business model (borrow short term, lend long term) is very similar to that of a bank, but by conducting its business through capital markets (rather than taking deposits) and being an off-shore entity it escapes the regulation that banks and finance companies are subject to. They can also often be kept off balance-sheet, escaping indirect restraints through regulation of the banks that set them up.