1. It's a ton of work, all the time. Let alone the office work, which becomes less important as a banker rises in seniority; just constantly being immersed in the world of finance is mentally and physically exhausting. Money never sleeps, so senior investment bankers rarely do either.
2. It's also reputation-conscious. Everything you say in public can harm your professional brand and deals you are working on, and give ammo to your enemies (and over the course of a successful IBK career, you will have made many of these).
Together, these facts make it unlikely a current ibanker will find the time to write a detailed answer to this question. So, you'll have to settle for me--a former very junior banker for a small boutique who lived the dream for a few years then skedaddled after getting the Associate promotion.
Investment banks' primary businesses are:
Raising capital for corporations, governments or other financial institutions (Stock or bond offerings, loans for LBOs, e.g.). Old powerful banks like Salomon Bros., JPMorgan and Goldman Sachs all got started doing this.
Making markets in all sorts of securities, from stocks and bonds to CDSs and futures (so-called "Sales & trading"). Every bulge bracket plus hundreds of smaller firms in many countries do this.
Advising on corporate restructurings, corporate finance and capital structure (Exactly what it sounds like). Most banks have groups dedicated to this, and some, like Rothschild and Lazard, are legendary for it.
Advising on mergers and acquisitions (Providing negotiating, analytical and other support during deals). Since all it takes is to do this well is a handful of bankers' time, this is the most profitable IBK business. Former Morgan Stanley M&A god Paul Taubman is currently one of the world's top M&A advisors--and he doesn't even have a secretary.
Helping institutions manage financial risk (Derivatives structuring, e.g.). In the recent past, some of this activity got a few