The previous posts were very accurate. I think that you might want to check out Howard Zinn's work, "A People's History of the United States." Of specific mention would be the chapter entitled "Robber Barons and Rebels." Part of the answer to this question would be located here. I think that the industrial capitalists of the late 19th Century can be seen as Robber Barons primarily because they were willing to do anything to consolidate their hold on the economic market. It is not that these individuals "played the game" of capitalism to its logical extension, but rather were instrumental in ensuring that they had government cooperation through instances of graft and influence. These individuals did what was needed in order to make sure that economic success would not be impeded through any means, confirming the idea that they could be seen as both "titans of industry" or robber barons who would do anything to ensure profit for their own benefit.
Many of these industrialists, too, amassed fortunes based on their exploitation of natural resources - Rockefeller with Standard Oil, Carnegie with US Steel, George Hearst with gold and silver mining just to name a few. So the term Robber Baron has also been used to describe the way they pillaged the Earth of resources - resources which could arguably have belonged to the entire public trust - in order to make their fortunes.
Remember this was a time of "laissez-faire" capitalism, where government did little if anything to regulate our nation's industries, so worker safety, worker pay, and the number of hours in a work day were all neglected or exploited so that the industrialist made more money. Retirement and medical care were never even thought of by worker or robber baron at the time.
So even though these robber barons often gave back millions in charity to the American public, some say that does not get them off the hook for the exploitative nature of their business models.
Significantly, the term Robber Barron comes from the medieval German lords who charged exorbitant fees on ships traversing the Rhine River. Like these feudal lords who had peasants working for them, the Robber Barrons of the United States gave little thought to the working conditions or the pay afforded those in their employment. The employees were exploited, often working long hours in dangerous mines and dirty, noisy factories.
In addition, such men as J.D. Rockefeller, Cornelius Vanderbilt, Andrew Carnegie, J.P. Morgan,and Jay Gould held monopolies in the country, which allowed them to set prices as they wished.
Many historians see this as really an unfair way to characterize the industrial capitalists of this era (like Andrew Carnegie), but there are reasons to call them this. The main reason is the fact that they were so much richer than all the other people of their time.
If you have workers who are getting a couple of dollars a day and the man who owns their factories gets millions a year, you can see why someone would say that man is a robber. He is getting hugely rich off of the work of other people (who remain poor). This seems like thievery.
Another reason to call them robber barons was because of the way they built their huge companies. People accused them of unfair business practices that drove competitors out of business, leaving the "robber barons" with monopolies or near monopolies. This, too, made it look like they were stealing from others to make themselves rich.
So, the basic idea is that these guys would do anything (exploit their workers, unfairly destroy competitors) to become rich.