From the Cardano website:
Following on from the Byron era, the Shelley era of Cardano is a period of growth and development for the network. Unlike the Byron era, which began at a single point in time when the mainnet was launched, the transition to Shelley is designed to achieve a smooth, low-risk transition without service interruptions.
The Shelley era encompasses the critical early steps in Cardano’s journey to optimize decentralization – and like any first steps, these will be gradual but significant. During the Byron era the network was federated, but as the Shelley era progresses more and more nodes will shift towards being run by the Cardano community. Once the majority of nodes are run by network participants, Cardano will be more decentralized and enjoy greater security and robustness as a result.
Shelley will also see the introduction of a delegation and incentives scheme, a reward system to drive stake pools and community adoption. As a proof-of-stake network, users stake their ada to participate in the network. Painstakingly designed using game theory and the latest research into proof-of-stake networks, the delegation and incentive scheme will allow and encourage users to delegate their stake to stake pools – always-on, community-run network nodes – and be rewarded for honest participation in the network.
Come the end of the Shelley era, we expect Cardano to be 50-100 times more decentralized than other large blockchain networks, with the incentives scheme designed to reach equilibrium around 1,000 stake pools. Current prominent blockchain networks are often controlled by less than 10 mining pools, exposing them to serious risk of compromise by malicious behavior – something which Cardano avoids with a system inherently designed to encourage greater decentralization. Not only that, but the entire Cardano network runs at a fraction of the power cost of equivalent proof-of-work blockchains, using the electricity equivalent of a single house, rather than a small country.
The Shelley era represents the natural maturation of the network, making it more useful, rewarding, and valuable for users new and old. It’s also about preparing for the future. Shelley will set the stage for a fully distributed network, and an entirely new application ecosystem with even greater things to come in the Goguen, Basho, and Voltaire eras.
A research paper that analyzes and discusses Ouroboros Praos: a provably secure proof-of-stake protocol that improves upon Ouroboros to secure Cardano against adaptive attackers.
A research paper that analyzes and discusses Ouroboros Genesis. Ouroboros Genesis further improves upon the Ouroboros proof-of-stake protocol by enabling stakeholders to securely join or rejoin the blockchain.
A research paper that describes an attack on a proof-of-stake blockchain without checkpoints, to illustrate the close relationship between transaction fees and rewards and the security properties of PoS protocols.
A research paper that explores two-party computation in a plain model in which no reliable channels are assumed (mirroring a real-world setting). We investigate the feasibility of communication-optimal, noise-resilient, semi-honest two-party computation, and devise an information-theoretic technique to that end.
A research paper that studies the probability change of a double-spend attack in a proof-of-work blockchain in relation to network parameters, and introduces the concept of threshold: a minimal adversaries ratio that provides a successful attack with probability.
A formal specification and executable model of the ledger rules introduced by the Shelley release, defining the functionality of the ledger on the blockchain and how to determine what constitutes a valid block via a collection of deterministic rules.
A formal specification that formalizes the definition of a valid block, and what is required for it to be added to the blockchain, in the scope of the Byron era and the transition to the Shelley era of Cardano.
An engineering specification that details the design of the necessary additions to Cardano to support and incentivize delegation.
A research paper that introduces and studies reward sharing schemes that promote the fair formation of stake pools in proof-of-stake blockchains with a large number of stakeholders, such as ada holders within Cardano.
This paper defines a simple, deterministic protocol for ledger consensus that tolerates Byzantine faults, where the protocol is executed by n servers over a synchronous network and can tolerate any number t of Byzantine faults with t < n/3.
This research paper introduces a new version of the Ouroboros consensus protocol which is provably UC-secure without assuming access to a global time functionality, addressing a longstanding shortcoming of proof-of-stake systems.