What is Stellar?
Stellar is a fully open-source decentralized blockchain that allows for near instantaneous (3-5 seconds settlement time) and almost zero cost cross-asset transfer of value using its native asset, called Lumens (XLM), as a bridge. This includes the ability to make payments using both crypto and fiat currencies as well as other types of assets through the built in decentralized exchange. Stellar is also an ICO platform which gives users of the network the ability to issue tokens. Stellar is backed by a non-profit foundation called the Stellar Development Foundation.
In other words, it is an open, distributed and global financial infrastructure network that anyone can use to very quickly and very cheaply move and exchange value worldwide.
For those interested in the technical aspects of Stellar, you can download the Stellar Consensus Protocol whitepaper from Stellar.org:
See videos and articles below for more.