Investment bankers are bankers who work in investment banks. Investment banks play a different role than what we usually think banks do in the US. In the US, we see banks as places in which we deposit our money. We can deposit money there and we might be able to borrow money there as well. However, these kinds of banks are typically called retail banks and commercial banks. They are different from investment banks.
Investment banks do not take deposits from the public. What they do is to help large customers such as governments and big businesses to achieve their goals. Investment banks do things like helping corporations to raise money by selling bonds. They help businesses that are trying to merge. They do other kinds of investment business such as creating and trading in derivatives. They can also work the opposite side of the markets by helping investors find places to put their money.
Investment bankers, then, engage in all of these activities. They involve themselves in trying to sell bonds or to find financing for mergers. They work on helping large clients invest. What they are doing is very different from the job of taking deposits and making relatively small loans, which is what retail and commercial bankers do.