What exactly is a trust? In my history class, we're studying the Industrial Revolution and how Rockefeller came to dominate the oil industry with his Standard Oil company. According to our textbook, a man named Samuel Dodd (who was Standard Oil's attorney) came up with the trust in 1881. But our textbook does a very poor job of explaining what exactly a trust is or why it helped Rockefeller to monopolize his business and basically dominate the oil industry. I've done a bit of research myself, and I'm not really good with economics. Could someone please explain, in very simple layman terms, what a trust is and what it does?
Wikipedia has an article worth reading.

An American trust was typically a combination of companies operating effectively as an entity with dominant market power, so perhaps somewhere between a cartel and a monopoly.  

Trusts originally took their names from the use of trust law to establish them, but later the term was used for all large companies and groups of companies which used their market position to take advantage of their suppliers and customers and to drive competitors out of business.  The most famous was Standard Oil.

This meant that when the US legislated to regulate or dismantle trusts, it call this antitrust law, where in Europe it would be called competition law.