What is a trust? What does the Beef Trust control?

In this context, a trust is "a combination of firms or corporations formed by a legal agreement; especially: one that reduces or threatens to reduce competition."

http://www.merriam-webster.com/dictionary/trust

Packingtown was a trust in which numerous entities combined under one major conglomerate. In this case, the conglomerate was the Beef Trust.

"He had learned by this time that Packingtown was really not a number of firms at all, but one great firm, the Beef Trust." (Page 120)