A 360° photo or video covers 360 degrees of view. They are usually viewable in your web browser by clicking and dragging around, on a smartphone by dragging or tilting, and even sometimes through Google Cardboard or a VR headset (like Vive).
Below is an example of a 360° video available on YouTube. Click and drag to move around.
YouTube - Hundreds of thousands of 360 videos. Start exploring with their VR Channel, or try applying the search filter “360°”
Sesqui - a Canadian 360 video company
News/Journalism - Immersive reporting from the world's major news outlets
NYTimes - The Daily 360
Al Jazeera - Contrast VR
Note: All videos available on YouTube are also viewable on Vive through YouTube app