which command prints the most precedent application on the PATH whereas
where prints all matching applications on the path.
The difference manifests when you have side by side instances of an application installed and all set on the path. Strictly speaking which by itself returns the instance resolved first on the path left to right, where returns ALL found instances on the path resolved left to right. If you want to see what I'm talking about, on a Windows 7 or above system type:
where notepad you may see two paths returned
The real equivalence is
which -a someApp ===