From the project site:

Serum is a decentralized exchange (DEX) and ecosystem that brings unprecedented speed and low transaction costs to decentralized finance. It is built on Solana and is completely permissionless.

• On-chain Orderbooks: Full limit orderbooks with an interface familiar to that of centralized exchanges
• Speed: Sub-second trading and settlement
• Cost: Lower transaction costs than any other DEX: $0.00001 per transaction