Before Starknet, you might have heard of StarkEx. StarkEx and StarkNet are both products of Starkware, a company building layer 2 solutions for Ethereum using zk-rollups.
StarkEx is an application-specific scaling engine that provides support for multiple dApps such as dYdX, Immutable X, and Sorare. StarkNet, on the other hand, is a general-purpose layer 2 network that allows anyone to deploy their smart contracts without permission.