The buy side comprises the investing institutions such as mutual funds, pension funds and insurance firms that tend to buy large portions of securities for use in managing portfolios. In other words, they are firms that are not dealer banks. The buy side is the opposite of the sell side entities, which provide recommendations for upgrades, downgrades, target prices and opinions to the public market.
Buy side analysts often work for a mutual fund or pension fund. They provide research and recommendations exclusively for the benefit of the company's own money managers rather than for individual investors. Sell-side recommendations are meant for the public; buy-side recommendations are internal for the firm.