Remote Management vs. Screen Sharing?
By default, Mac OS uses VNC combined with a few extras (e.g. auto-discovery via Bonjour) when you use Screen Sharing, and in most cases this is all an average user needs.
For system administrators however, there is usually a need for more control over the computers they support; therefore Apple provides "Apple Remote Desktop" (ARD) which offers a lot more control mechanisms (like terminal sessions, remote installation of software, remote shutdown, remote execution of automator scripts) and this is possible with groups of many computers at the same time. For this feature to work, SSH and remote management needs to be activated.

So in short: Screen Sharing is roughly equivalent to VNC, while Apple Remote Desktop is a "high-end" remote administration software which requires "Remote Management" to be turned on. There is also a difference in how incoming screen sharing requests are handled: with "Screen Sharing" enabled, if an observer connects with a user account that's different than the one currently logged in and clicks Share Display, the user needs to confirm before the screen is shared. With "Remote Management", the screen is immediately shared without a prompt appearing.