Per multiple media reports, sites supporting services such as Twitter, Reddit, Airbnb, Spotify, and Reddit were among those that were impacted.
A DDoS attack is when someone floods a site with traffic or requests in order to overload its servers and make the site's services unavailable to people legitimately trying to use it.
No group has yet come forward claiming responsibility or providing a motive.
Because of the popularity of the sites affected by this morning's DDoS attack, this could just be an example of hacker hubris, a "because I could" sort of thing.
UPDATE 2: A group has claimed responsibility. Click Here for a follow-up story.