To start with, a merger occurs when two companies combine to form a single company. A merger is very similar to an acquisition or take-over, except that in the case of a merger existing stockholders of both companies involved retain of a share interest in the new corporation. By contrast, in an acquisition one company purchases a bulk of a second company's stock, creating an uneven balance of ownership in the new combined company (wisecGeek what is a merger).
In many cases, a firm will merge with another in order to benefit from what the other company has. Examples are mobile companies Sony and Ericsson which has both merge together to form Sony Ericsson. This integration has made them one of the leading mobile company's