The sell side refers to a brokerage or firm that manages individual accounts and makes recommendations to the clients of the firm. Sell side analysts are those who issue recommendations such as "strong buy," "outperform," "neutral," or "sell." The brokerage benefits from these recommendations because whenever a client trades stock, the brokerage gets a commission.
Sell side analysts are paid by and ultimately answer to the brokerage, not the clients. Their recommendations are not specific to one client but are aimed at all or any of the firm's clients.
Also see: buy side