In a nutshell, roll ups are separate blockchains where all the computation is done to decongest the Ethereum Mainnet.

Transactions are highly compressed before being posted on the Mainnet, in order to optimize space blocks.

There are two types of roll-ups that distinguished themselves on how the information posted on L1 is verified: Optimistic Roll-ups (OR) and Zero-knowledge proof Roll-ups (ZKR).

In OR, it’s pretty simple, we are... optimistic: we assume that all the transactions are correct and that there are no malicious validators trying to post fake stuff into Ethereum.

Because batches (compressed transactions) are posted without verifications, those roll-ups would increase transaction throughput by 10x-100x to the great satisfaction of Ethereum users.

The other side of the coin is the length of the protest period: Validators on OR have a week to spot a malicious transaction and publish a fraud-proven.

This time range lowers the user experience by making withdrawals from roll-ups very lengthy - which is not ideal.

As a user, you always want to have your funds very easy to move from a protocol to another.

In ZKR, every batches posted on Ethereum is previously verified thanks to a validity-proven.

Validators create a zero-knowledge proof for each bundle of transaction so people can easily verify that there’s no malicious actors in the roll-ups.

Zero-knowledge proof is a mathematical way to prove something to someone without revealing the actual stuff.

Thus, with ZKR, you rely on maths, not on validators. (cc ZkSync)

The cons is that a validity-proven require a lot of computation, so powerful hardware equipment, which can slowing down the adoption of such a technology or lead to more centralization.

The leaders for OR are Arbitrum and Optimism.

And for ZKR, ZkSync and Starkware are leading the way.