Blockchain is a decentralised technology system that stores data in a block structure with multi-party maintenance and uses cryptography to guarantee data transmission and access.
I've read a vivid analogy of its definition from What is Blockchain Technology: A and B lived in a same village and one day A borrowed money from B. B asked the village head C to be the witness to record the deal in case that A repudiate the debt. That might lead to "centralisation" because C is the only one with the rights to modify the account book. Gradually, villagers worry that C would embezzle money, so everyone gets an account book and they will be informed of each transaction by announcement. That is "decentralisation". Blockchain is similar to a public account book which allows everyone to record data.
The three most important features of blockchain technology are: decentralisation, tamper-proof and traceability. The first one is explained in the above example. Tamper-proof means that the information on the blockchain cannot be modified at will and traceability means all the data sources will be visible and clear on the blockchain. Because of this, it is expected to benefit many industries, such as the e-commerce. It can secure the online payment and allow people to keep track of the supply chain and logistics flow, thus preventing the counterfeit issues.