Form 1099-B is used to report proceeds from the sale of investments - stocks, mutual funds, bonds and other financial instruments (e.g. options). In many cases, they include cost basis information and calculate your taxable gain; in some cases only the proceeds are reported and you have to enter the cost basis.
They represent either a taxable gain or deductible loss, so you must report them on your tax return in the Stocks, Mutual Funds, Bonds and Other interview of Personal Income > Investment Income.
