This article refers to the imToken Help Center Basic concepts about Layer 2.
Rollup is one of the layer 2 solutions that perform transaction execution outside the main Ethereum chain (layer 1), but post the compressed data on layer 1.
According to the different schemes to ensure the validity of the compressed data, Rollup can be divided into ZK Rollup and Optimistic Rollup.
ZK Rollup: ZK Rollup bundles hundreds of transfers off-chain and generates a cryptographic proof, known as a SNARK (succinct non-interactive argument of knowledge). This is known as a validity proof and is posted on layer 1.
Projects that use the ZK Rollup solution include: zkSync, etc.
Optimistic Rollup: One type of Rollups which optimistically believes that the data posted to the Ethereum mainnet is correct, and if anyone observes that the data is wrong (i.e., there is fraud), they can issue a challenge during a period. Therefore Optimistic Rollup is also called fraud proof. In this way, it is guaranteed that the data finally posted to the Ethereum mainnet is valid.
Projects that use the Optimistic Rollup solution include: Arbitrum, Optimistic Ethereum, etc.