RGB as a smart contract system described in the LNPBP-11 standard is able to work on top of the Lightning Network and designed with confidentiality and scalability in mind. It's created & maintained by the LNP/BP Standards Association. More information about the Association's activities can be found here.
What is possible with RGB:
Issue digital fungible assets, like stock, bonds and other forms of securities;
create different forms of collectibles (non-fungible assets);
create and manage sovereign/decentralized identities and reputation systems;
create and maintain provably-unique history of some events that may be used for audit with well-controlled partial disclosure of the data;
design and run other forms of arbitrary-complex smart contracts
As a smart contract system RGB is quite different from previous approaches, both Bitcoin-based (Colored coins, Counterparty, OMNI) and non-bitcoin (Ethereum, EOS and others):
RGB separates the concepts of smart contract issuer, state owners and state evolution
RGB keeps the smart contract code and data offchain
RGB uses blockchain as a state commitment layer and Bitcoin script as an ownership control system; while smart contract evolution is defined by off-chain schema and Turing-complete scripting system using Simplicity language
More about these concepts can be read in this presentation.
Briefly, RGB smart contracts operate with client-side validation paradigm, meaning that all the data is kept outside of the bitcoin transactions, i.e. bitcoin blockchain or lightning channel state. This allows the system to operate on top of Lightning Network without any changes to the LN protocols and also gives a foundation for a high level of protocol scalability and privacy.
As a security mechanism RGB uses single-use seals defined over bitcoin transaction outputs, which provides ability for any party having smart contract state history to verify its uniqueness. In other words, RGB leverages Bitcoin script for its security model and definition of the ownership and access rights.
Each RGB smart contract is represented by some genesis state, created by smart contract issuer (or, put simply, issuer) and a directed acyclic graph (DAG) of state transitions kept in form of client-validated data (i.e. this data is not stored on blockchain or within LN transactions/channel state). The state is assigned to unspent bitcoin transaction outputs, which defines them as single-use seals. The party that is able to spend corresponding transaction output is named a party owning state: it is a party that has the right to change the corresponding part of the smart contract state by creating a new state transition and committing to it in a transaction spending the output containing previous state. This procedure represents closing of a seal over state transition, and a pair of spending transaction and corresponding extra-transaction data on the state transition are named witness.
State transition assigns state to a set of defined single-use seals. Each smart contract may maintain different forms of state and define different kinds of single-use seals with different validation rules. Additionally to this, state transition may contain different metadata and Simplicity scripts, defining parts of its business logic.
Which types of state, seals, metadata and which script extensions are allowed within state transitions is defined by schema. Thus, schema can be seen as validation rules for client-side validation; schema is always defined by the issuer in state genesis. Schema also may contain Turing-complete Simplicity scripts defining parts of the business logic for client-side validation.
RGB operates in "shards", where each contract has a separate state history and data; different smart contracts never intersect in their histories directly. This allows another level of scalability; and while the therm "shard" is incorrect, we use it to demonstrate that RGB actually achieves what was planned to be achieved with "Ethereum shards".
While being separately maintained, RGB contracts may interact via RGB DEX protocol over the Lightning Network, allowing multiparty coordinated state changes, which, for instance, enables functionality like DEX over Lightning etc.
Thus, by their abilities RGB smart contracts go beyond what is possible with Ethereum-like smart contract system, providing more layered, scalable, private and safe approach, where the ownership of the smart contract state is separated from the smart contract creation.