An Object Variable can store a reference to anything derived from UnityEngine.Object.
Many things in Unity are derived from UnityEngine.Object, so this opens up a huge array of possibilities!
Object Variables are more advanced than other variable types since you need a deeper understanding of the Objects you're using.
Playmaker lets you poke around inside Objects using Get Property and Set Property, but it's a good idea to consult the Unity Scripting Reference to get more details on a given Object type.
For example, if you want to manipulate an AnimationClip stored in an Object variable you might look at http://docs.unity3d.com/ScriptReference/AnimationClip.html to read about its properties.
See Also
