Centralised networks rely on a single entity (a private company or a government, for example) that has control over every aspect of the network. This authority has the power to censor content, manipulate the flow of traffic, and even restrict access to the system entirely.
All these factors mean that the use of a centralised network requires users to place their trust in the authority that is in control.
In decentralised networks there is no central authority, no single entity has more control than any other. The absence of a single point of failure means they are much more resistant to attack, manipulation or suffering unexpected downtime. The power is distributed across many participants, which protects against collusion and conspiracy, ensuring that the network remains secure without the need to trust a centralised authority.
Blockchains, by their nature, are good examples of decentralised networks. No individual has the power to affect the system as a whole thanks to the consensus mechanisms (such as Proof of Work or Proof of Stake) that secure and validate the networks.
Decentralised blockchain networks are therefore the perfect infrastructure for the use of cryptocurrencies, as they can be freely transferred, traded or invested without needing to trust in a single centralised authority.