Wash sale rules are designed to prevent taxpayers from claiming capital losses on sales where the taxpayer really didn't dispose of the underlying asset. Let's say that I bought 100 shares of CMGI stock for $75 per share, totaling $7,500. CMGI has plunged to $2.00 per share. I would love to take the loss on the stock but I am still convinced that I want to hold on in case it goes back up.
If I sell the 100 shares of CMGI stock on November 1 to take the loss and buy back 100 shares on November 15, the IRS position is that I am not entitled to claim the loss because I still own the stock. The wash sale rule says that I can not buy the same asset 30 days before or after the sale date for the transaction that I want to claim the loss for. With that in mind, I can not buy those 100 replacement shares of CMGI from October 2 through November 30 if I want to claim the loss on the shares sold on November 1.
To work around those rules, I would buy 100 shares of CMGI on October 1, and then my sale on November 1 of the original (old high cost basis shares) CMGI stock at a loss would be allowable, and I would still at the end of the day have 100 shares of CMGI.
Another way to deal with this is to buy "similar" securities. For example I could sell my Vanguard Growth Fund shares and replace them with Fidelity Growth Fund shares. "Substantially Identical" securities (if you can define that) will not work.