it is where an investment bank buys and sells the products like stock, bonds, commodities, foreign exchange etc. Traders in the sales & trading division act on the banks account, using the firm capitals to place a bet where they see the opportunities to make money or acting on client instructions. Research analyst write the reports on expected earnings in companies and the industry they follow.
The middle office usually deals with financial risk controls and reporting. There’s a lot of back and forth between the front office and middle office to ensure that the bank's trading and underwriting isn’t risking too much of its capital. Common tasks in the middle office are compliance, financial controls, corporate strategy, corporate treasury, and managing risk.