Arbitrum is a technology suite designed to scale Ethereum.
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Arbitrum is a set of tools that helps make scale Ethereum. With Arbitrum, you can do the same things you do on Ethereum, like use web apps and make smart contracts. But, your transactions will be faster and cheaper.
The main product, called Arbitrum Rollup, is a rollup protocol using Ethereum-level security. The basic idea is that an Arbitrum Rollup chain runs as a sort of sub-module within Ethereum