Here's a detailed article on this topic by CNBC:
cnbc .com/2019/06/18/the-difference-between-bitcoin-and-facebooks-libra.html
In my view, the biggest difference between Facebook's Libra and the Bitcoin is that while Libra is a Stablecoin, Bitcoin is not!
Libra is built on a secure, scalable, and reliable blockchain. It is fully backed by a reserve of real assets. A basket of bank deposits and short-term government securities will be held in the Libra Reserve for every Libra that is created, building trust in its intrinsic value. While bitcoins are created as a reward for a process known as mining in the computer. They can be exchanged for other currencies, products, and services.