The STS-118 mission will deliver and assemble the starboard S5 truss segment to the International Space Station, as well as External Stowage Platform 3, and a replacement Control Moment Gyroscope (CMG). The mission also will carry the Spacehab (Logistics Single Module).
STS-118 will be the final flight to include the Spacehab module, a pressurized aluminum habitat that is carried inside the cargo bay. The Spacehab has a capacity of 6,000 pounds, and will carry a variety of cargo and research projects, including supply materials for the ISS. The Spacehab will return with approximately 3,000 pounds of cargo, including the MISSE PEC, a Department of Defense payload that had been installed on the ISS. Launched in July 2006, the MISSE PEC contains over 850 materials specimens that will be studied after long-term exposure to the environment of space.
Taken from Wiki.