A layer-2 is a term used to describe a set of scaling solutions. A rollup is an off-chain network built on top of the layer-1 that executes transactions off-chain, bundles these transactions, and posts the bundled transactions as a single transaction on the layer-1 chain.
Fuel is designed to run a modular execution layer, a configuration similar to what we call rollups or layer-2s on Ethereum today. Rollups typically use an optimistic or zk-configuration for validity or transaction arbitration. The Fuel technology is agnostic to either of these and can utilize either as a validity or fraud-proving system.
Layer-2s and rollups are primarily designed for monolithic blockchain stacks, which means they are typically not optimized for large amounts of layer-1 bandwidth potential unlike Fuel, which is uniquely configured to handle this potential.