Stellar (and Ripple) are distributed ledger and exchange. The big difference from Bitcoin is that it records credits from gateways in multiple currencies. It can transfer money in regular currencies and exchange different currencies.

The other big difference is that consistency is not maintained by mining but with consensus between trusted servers. This makes theoretically scale to larger volumes and better latency while not being fully distributed.

The Bitcoin cryptocurrency people hate it because it uses fiat currencies and is more centralized. Most people will probably prefer Stellar since it can interface with existing bank accounts and monetary systems. It can be complement to Bitcoin as it can act as a distributed exchange.

Stellar is just a fork of Ripple:

https://news.ycombinator.com/item?id=8115272