Obama became the 44th president of the United States, and the first African American to hold the office. He served two terms and was succeeded by Donald Trump.
Other notable events on January 20th:

In 1981, the Iran hostage crisis, where Iranian militants held 52 American citizens hostage for more than a year ends with the release of the hostages.

In 1945, U.S. President Franklin D. Roosevelt is sworn in for an unprecedented fourth term in office.

In 1892, the first official basketball game is played in Massachusetts, by students of the game’s inventor, James A. Naismith.

In 1778, British explorer James Cook  becomes the first European to visit Hawaii after landing at Waimea, on Kauai island.