Sometimes I find out that application I'm using at the moment has > (greater than) letter on the left of its icon in Unity Launcher instead of standard triangle. But in Ubuntu Documentation, there isn't this shape mentioned. What does presence of this shape mean?
It means the corresponding application is running, but its window is not on the current workspace :)
on the current workspace:
on another workspace:
If you click on the icon, the workspace usually switches to the one with the application's window on it.