Person A is a football player and makes $1 million in salary per year. This is all taxed progressively, meaning that Uncle Sam takes off 39.6% of most of his income. As a result, he pays close to $400k in taxes.
Person B is an investor. He invests in Applied Materials, and they have a very good year. His stock is now worth $1 million more than it was at the beginning of the year. Some of it, he sells - he's gotta eat, after all. That sold stock is taxed like income. Note that as long as he isn't living like a rockstar, he probably didn't sell enough to put him into the top tax bracket; he's just paying his expenses.
The remainder is taxed at about 20% - capital gains is taxed at a lower rate. Thus, because he can control how much of the stock he sells, Person B ends up paying somewhere around 60% of the tax that Person A pays.
This system screws over people who make most of their income through salary and benefits those who make most of their "income" through investments.