There are two sides of Wall Street - Buy - and - Sell. Duh. Why are there two sides like this? Because the antagonistic" nature of pricing for securities works, for the most part. The "sell-side" comprises the stock broker community, people who sell used companies. That august group includes investment bankers, capital markets makers and more or less all of the people who wear ties at Goldman Sachs.
The people who don't wear ties at Goldman are a sloppy division which runs mutual funds for Goldman or GSAM - Goldman Sachs Asset Management. The "buy-side" are people like mutual fund managers, hedge fund managers, pension fund managers. "Fund managers" is the key phrase there.
They are the ones responsible for the money. They have to report to shareholders whether they have made or lost money and how much risk they took to get there, for hedge fund managers, especially. Unlike the broker community of sell-siders, the buy side has a broader fiduciary obligation to their investing clients.