Definition
Privacy is the ability to determine what information about oneself is shared with whom. It is distinct from anonymity (being unidentifiable), security (resisting attack), and confidentiality (specific information not being disclosed). A person can be private without being anonymous (sharing real identity selectively), or anonymous without being private (engaging in extensive disclosure under a pseudonym). VPNs primarily serve privacy by limiting what ISPs, networks, and websites can observe about a user's online behaviour.
Privacy vs Security vs Anonymity
Security is about resisting active attack: keeping passwords secret, preventing malware, blocking unauthorized access. Privacy is about limiting passive observation: not having your activity logged, not having data sold, not having patterns analyzed. Anonymity is about not being identifiable at all. A VPN contributes to privacy strongly, to security marginally (by preventing eavesdropping on hostile networks), and to anonymity only partially (because the VPN provider sees what the ISP used to see).
Privacy as a Trade-Off
Every VPN purchase is a privacy trade-off: shifting trust from your ISP to your VPN provider, and from your home country's data laws to wherever the VPN is incorporated. The trade is usually favourable but not always — a free VPN that monetizes by logging traffic is worse for privacy than a regulated ISP. The framework for evaluating any VPN provider is: who do you trust more with your browsing patterns, and what evidence supports that trust?