It is important to remember that although "The Jungle" is a novel, Upton Sinclair was a "muckraker". This form of journalism founded during the progressive movement in the United States. This was a time when people were questioning the impact industrialization had on this country, at the same time when literacy rates were on the rise. Today we refer to this tactic as raising awareness. Sinclair was what we would call today an investigative reporter, he actually went into the meat packing factories, worked and witnessed the unsanitary conditions along with what the industry was actually selling Americans. The main point of the novel was to expose the harm that industry and big business were causing Americans all for the sake of profit.
If you are looking for themes, two of the main ones are naturalism, the pursuit of the American Dream, and socialism. Sinclair demonstrates naturalism when he portrays the characters as animals who cannot control themselves. Jurgis looses his temper on several occasions and lashes out. He also becomes an uncontrollable alcoholic for a period. All of the characters are victims of their environment (Stanislovas being eaten by rats, etc).
The character pursuit of the American Dream is also a theme. The family immigrates to America for a better life. They believe they can have a house and jobs, but they realize quickly that the dream is an illusion.
Sinclair's answer to the failed American Dream is socialism which is presented in the last section of the novel. Jurgis is attracted to the philosophy because he can see how it would have helped his family. It attracts the common worker with promise of equalizing everyone. The novel ends on a hopeful note as Jurgis joins the socialist movement.