Shorting is selling first and buying later. The idea is to sell high and then buy low. It can be a bit risky. Since there is no cap on how high a stock can go you can loose more than you invest. Still, as part of a portfolio, using short positions can possibly be a useful strategy at times. You can use shorting to do things like hedge against existing gains (without selling those positions and incurring taxes).
Business Week had an article on Shorting for the 21st Century using inverse funds. These are mutual funds that are structured to behave as short positions – that is to go up if the target portfolio goes down in value. One advantage of using these funds (at this time they are all ETFs – exchange traded funds, I believe) is that you losses are limited to your investment. You do incur additional expenses charged by the fund however.
Experienced investors may find value in exploring the use of inverse funds. Some funds are engineered to move 1 for 1 with the market (that is the fund increases 1% for every 1% decline in the index) and some are engineered to move up 2% for every 1% decline – which also means they go down 2% for every 1% gain in the actual index. Index funds can also be used in retirement accounts (where shorting is not allowed).
Most investors need much more experience and to do a great deal of reading before they would be ready to try these funds. Since markets general go up over time and timing the market is extremely difficult it is unlikely novice investors will succeed in trying to guess right. The usefulness is mainly as a hedging strategy when the investors has determined the portfolio could benefit from a partial hedge.