A light node stores only block headers, not the full blockchain. It downloads a small chain of headers and relies on full nodes to provide transaction details when needed. That makes it fast to sync and cheap to run. A light node can run on a phone or a laptop with modest storage. The trade-off is trust. A light node cannot independently verify every transaction. It asks full nodes for proof and checks that proof against the headers it holds.

Light nodes use simplified payment verification, or SPV, to confirm that a transaction is included in a block. The method checks the Merkle branch that connects the transaction to the block header. It is secure as long as the light node connects to at least one honest full node. If all its peers lie, the light node can be fooled. That is why light clients connect to multiple peers and use random sampling. Ethereum's light client protocol is still developing, but the goal is to let phones and browsers verify the chain without downloading hundreds of gigabytes.

Light node characteristics

• Stores headers only — small storage footprint
• Fast sync — minutes instead of days
• Relies on full nodes — cannot fully validate independently
• Good for mobile and browser wallets

A light node is a compromise. It trades some security for convenience. For most users, that trade is worth it.