Where do I get the latest version of Craftbukkit?
This is where you can always find the latest recommended version of Craftbukkit. Please keep in mind that these change quickly and from time to time may get ahead of the plugins compatibility. It is always best to use recommended builds, other Craftbukkit builds may have errors or compatibility issues that you are not aware off, and most plugin developers, design their plugin to the Recommended Builds only.
How do I tell the version of Craftbukkit and Essentials?
If you downloaded CB from the link above then you should know your version. If you look at the console it will also tell you the version of all plugins.
The commands "/version" and "/version Essentials" tell you the version while in game.
How often should I update my plugins and Craftbukkit?
Different people do it different ways, but you should always update CB any time you update any plugins. Plugins are updated based on the newest build of CB, so installing a new build of a plugin with an old CB will most likely cause it to break.