Definition
A VPN server is the machine on the provider side of a VPN connection. It accepts connections from clients, terminates the encrypted tunnel, decrypts the inner traffic, and forwards it on toward whatever the user actually wanted to reach. Replies follow the reverse path. The IP of the VPN server is what destination websites see as the visitor's address, so the server's location effectively becomes the user's apparent location.
What Determines Quality
A few characteristics distinguish good VPN servers from mediocre ones. Hardware capacity affects throughput; oversold servers lag at peak times. Network connectivity (the upstream bandwidth and peering arrangements) affects performance to specific destinations. Architecture (RAM-only versus persistent disk) determines log-retention guarantees. Geographic distribution (how many countries, how many cities) affects users' ability to choose the right exit. Security hardening determines resistance to compromise.
Choosing One
VPN clients usually offer auto-selection (often based on lowest ping) plus manual override. The right choice depends on the goal: lowest ping for general use, specific country for streaming, P2P-optimized for torrenting, obfuscated for restrictive networks. Server load matters too — a less-loaded server is usually faster than the closest one when the closest is overloaded.