What's web 3.0?
Web 3.0 was first mentioned by co-founder and former CTO of Ethereum, Gavin Wood. It was used to refer Decentralized Applications run on Ethereum Blockchain.
You can read the full original article ĐApps: What Web 3.0 Looks Like here
Web 3.0 = Ethereum (or other Smart Contract Blockchain) + Swarm/IPFS (peer-to-peer method of storing and sharing hypermedia in a distributed file system) + JS/CSS ...
You will not need a central server (backend server) to run a Web 3 Application (DApp/Decentralized Application), all assets and data are stored in a peer to peer network