Every proxy list labels entries by anonymity level, but the terms aren't self-explanatory unless you know what a server can actually detect about your connection.
Transparent
A transparent proxy passes your real IP address along in request headers even though it's technically routing your traffic. The destination server can see exactly who you are, so it provides no privacy benefit — it's mainly useful for caching or content filtering, not anonymity.
Anonymous
An anonymous proxy hides your real IP but still identifies itself as a proxy through headers like X-Forwarded-For or Via. The site knows a proxy is being used, just not which IP is actually making the request.
Elite (high anonymity)
An elite proxy hides your IP and doesn't announce that a proxy is in use at all, making your traffic look like a normal direct connection. This is the level most people mean when they say they want an 'anonymous proxy,' and it's the hardest tier for free lists to actually deliver reliably.