A "trust" in the Gilded Age setting of Upton Sinclair's The Jungle was an organization of businesses whereby they tried to control an entire market or industry by an agreement between several different owners within that industry to control production and fix prices.
In this manner, they can act like a monopoly and every owner in the trust can make larger profits. In Sinclair's day, especially in the socialist circles he traveled, to set prices in this manner was immoral, and led to exploitation of workers and consumers alike. To him, this was the point of the book, that trusts needed to be ended and socialism adopted for there to be social justice of any kind.
You can find out more about this question in Chapter 11 of the book.
Basically, the Beef Trust is what Jurgis calls all the companies that run packing houses in Packingtown. He thinks of them as one big company because the bosses all get together now and then to make policies that all the companies will stick to. By doing this, they made it so they would not compete with one another. They made it, for example, so there would only be one wage for workers so a worker could not go to another packing house and get better wages.