Here is a video that explains it:

Basically Justin Sun (TRON blockchain) a few weeks ago bought steemit INC., many people was concerned with the centralization activities (which oppose what blockchain technology wants: decentralization) that he did after buying the steemit INC. (steemit INC. holds about the 20% of all the STEEM).

So a lot of people decided to create an equal blockchain (HIVE) in which Justin Sun could not have this power of centralization, and remain a decentralized community. Since the creation of HIVE a lot of users have moved there, and Justin Sun has done even severe actions towards centralization in STEEM: like censor some posts that he didn't like, even froze some witnesses accounts.

If you had an steemit/steem account before the creation of HIVE, you have a HIVE account also (with the same amount of tokens you had at steem). Just go to peakd.com or hive.blog and enter with the usual username and password as always you did with steem/steemit.

If you'd like to buy hive you can at bittrex and ionomy