The piece of software that transforms a computer into a “node” in the Bitcoin network. Clients help in the generation of private keys, security, and payment. Clients can be “full” or “mobile”, with full clients storing the entire blockchain whilst mobile clients only store parts of it.
Running a full Bitcoin client is also the most secure and private way to use Bitcoin. It ensures that all the consensus rules are followed, and ensures that no one else can know which addresses are being used by you.