Proxy lists and documentation throw around a lot of terms that aren't always explained. Here's a quick reference for the ones that come up most.
Proxy server
A server that sits between you and the sites you visit, forwarding your requests on your behalf so the destination sees the proxy's IP instead of yours.
HTTP / HTTPS / SOCKS5
Different proxy protocols. HTTP handles standard web traffic, HTTPS adds support for encrypted connections, and SOCKS5 is protocol-agnostic, handling any type of traffic, not just web browsing.
Anonymity level (transparent / anonymous / elite)
How much a proxy reveals about your real identity. Transparent proxies reveal your real IP, anonymous ones hide it but announce a proxy is in use, and elite proxies do neither.
Latency
How long a request takes to complete through the proxy — lower is faster, and it's affected by distance, server load, and the proxy's own connection quality.
Rotation
Automatically switching between multiple proxy IPs, either per request or on a schedule, to avoid overloading or exposing any single IP.
Datacenter / residential / mobile proxy
Categories describing where a proxy's IP comes from — a data center, a home internet connection, or a cellular carrier — each with different cost, speed, and detection-resistance tradeoffs.