steemit.com is just a user (web) interface to the Steem blockchain. You can think of the blockchain as a database and steemit.com as the frontend. Busy.org is just another frontend that reads the same database. There are many like this: utopian.io, yehey.org, fundition.io etc...
I've explained a little bit more in a previous witness update post:
https://steemit.com/budgets/@quochuy/witness-update-2018-03-30-server-upgrade
I'll be checking them out; thanks for explainingThanks @quochuy