Space Shuttle Endeavour lifts off with a crew of six on mission STS-134, “....the 134th shuttle mission, the 36th shuttle flight to the space station, and the 25th flight of Endeavour.” Endeavour launched at 8:56 a.m. on 16 May 2011 from Kennedy Space Center into a low and dark cloud deck.

An excerpt from a NASA press release explains the mission:

Endeavour’s 16-day mission will deliver the Alpha Magnetic Spectrometer-2 (AMS) to the International Space Station. AMS, a particle physics detector, is designed to search for various types of unusual matter by measuring cosmic rays. Its experiments are designed to help researchers study the formation of the universe and search for evidence of dark matter, strange matter and antimatter. Endeavour also will fly the Expedite the Processing of Experiment to Space Station (Express) Logistics Carrier 3 (ELC-3), a platform that carries spare parts that will sustain space station operations once the shuttles are retired from service. The mission will feature four spacewalks to do maintenance work and install new components. These are the last scheduled spacewalks by shuttle crew members. Shuttle mission STS-134 is the final flight for Endeavour and the second to last flight for the Space Shuttle Program.

UPDATE:
Visit the End of the Space Shuttle Program Special Section