Peter Gaži, research fellow at IOHK, works on the Ouroboros family of protocols. In this quick video, he gives an overview of how the versions of Ouroboros relate to one another.
Originally, there was the first Ouroboros protocol. This was published last year at Crypto 2017 and is the underlying protocol of the current implementation of the Cardano network.
Further versions came to exist to address other interesting, open questions and to improve security guarantees.
The next version to arise was Ouroboros Praos. This version gives stronger security guarantees in the presence of stronger adversaries. Stronger adversaries are described as those that are capable of delaying messages, have certain control of network and the delivery of protocol messages among honest party and have more control over corrupting honest parties that participate in the protocol.
Following Praos, the most recent version to come out of this family of protocols is Ouroboros Genesis. This version tells us more about how this protocol compose with other protocols that could provide more functionality on top of the blockchain running on this Proof-of-Stake system. It also addresses another important problem of bootstrapping from genesis. Ouroboros Genesis describes a secure way of how a party that wants to join the protocol later, can do so securely with only having a trusted copy of genesis block and without the need for guaranteed checkpoints.
Peter Gaži notes in this video that he is more and more convinced that Proof-of-Stake is the right approach to maintaining a blockchain, as opposed to ways that require physical resources such as Proof-of-Work, Proof-of-Space or other variants of these.