Why Is It That Marx Accerted That Capitalism Is Exploitation Of Man By Man?

This would need a very long answer in full, but the basic principle is quite brief. The idea is that, in capitalism, the worker has no control over the means of production (eg. In an old agricultural society s/he might have grown his own food; in the socialist ideal society, the factory etc would belong to its workers; but in our society, one person/ group owns the business, and another works in it. This means that he has nothing to sell but his labour.) Suppose the worker has to put in 5 hours in order to cover the cost of his food, clothes etc (in a rich society, the worker may have a high standard of living - exploitation doesn't have to mean starvation.) If he works 8 hours, the value generated by his labour goes to the factory owner - the capitalist.

There are a lot of websites where this is explained in more detail. One which has a very clear, lively approach is www.marxist.com