The main difference between centralised and decentralised systems is the way in which decision-making and control are distributed. In a centralised system, decision-making and control are concentrated in a single entity or group of entities, such as a central server or a central authority. In contrast, in a decentralised system, decision-making and control are distributed among the participants in the network, with no single entity or group of entities having complete control.
Decentralised systems, such as blockchain networks, are designed to be trustless, meaning that they do not rely on trust between participants in the network. Instead, they rely on cryptographic algorithms and protocols to ensure the integrity and authenticity of data and transactions, they rely on verification. A common phrase that sums up a blockchain ecosystems is "Dont trust, verify".
In a blockchain network, all participants have a copy of the ledger, which ensures that there is no single point of failure. The ability for everyone in the network to build a common consensus due to verified-only transactions on the blockchain, establishes the trust and motivation to use such a decentralised system.