The bulge bracket banks are the very largest investment banks. They operate globally, dealing with all, or almost all, sectors, and in a wide range of businesses.
The bulge bracket banks are big enough to handle large transactions, with both the expertise and the financial resources needed to take big risks (such as underwriting very large issues).
The main problem posed by the bulge bracket banks is, that because they operate in many businesses for many different clients, they often face conflicts of interest. Although chinese walls and other measures and other measures are supposed to solve this problem, their effectiveness is doubtful. For this reason, some clients prefer to use smaller investment banks and brokerages. These may be substantial regional investment banks, or smaller boutiques.