Halo 2 is the name of both a leading-edge zk-SNARK theory and its implementation. It was developed by the team behind Electric Coin Co., the inventors of Zcash. Halo 2 is a successor to Halo, and both systems are inspired partly by the zk-SNARK theories PLONK and Sonic, among others.
Orbis Labs chose Halo 2 as the basis for our zk-SNARK implementations due to the following considerations. In no particular order:
Relatively stable and mature. It is audited, production ready, and used at scale in production.
Efficient proving algorithm.
Relatively small proofs which are not very computationally difficult to verify.
Rich ecosystem of tooling, SDKs, and reusable circuit code.
Does not require a trusted setup process and has no trust assumptions. Verifiers need not trust provers or any third party.