What did Lewis and Clark do on their expedition?

The expedition's goal as stated by President Jefferson was "to explore the Missouri River, & such principal stream of it as, by its course & communication with the water of the Pacific Ocean may offer the most direct & practicable water communication across this continent, for the purposes of commerce." In addition, the expedition was to learn more about the Northwest's Natural Resources, inhabitants, and possibilities for settlement. In the end, they were able to find and categorize 122 new animals and 178 plants, mapped the geography, and achieved friendlier relations with the natives. However, they did not find an all water route to the Pacific Ocean.

Before Lewis met up with Clark, he began the expedition on August 30, 1803 in Pittsburgh PA. Lt. William Clark would offer to join Lewis on the expedition weeks later on October 13, 1803 at Camp Dubois (in present-day Indiana).

The team departed St. Louis in May 1804 heading up the Missouri River. They would spend their first winter at Fort Mandan at the present site of Bismarck, North Dakota. In spring 1805, they continued to the headwaters of the Missouri River, struggled across the Continental Divide, and headed west along the Salmon, Snake, and Columbia Rivers to the Pacific. They returned to St. Louis the following year.

If you don't believe that the expedition really began in Pittsburgh instead of St. Louis, please refer to the related link which will lead you to the first entry of the Lewis and Clark Journals.