In the Selection preferences, there’s a Selection Change Command text box where you can type in a command to run when the selection changes.

Here are two important points about the Selection Change Command:

1. Use the VBScript syntax to enter the command. It doesn’t matter what the current scripting language is, the preference expects VBScript (so something like Application.LogMessage( “Hello %s” % Application.Selection(0) ) will end up logging an error when you select something).
2. The Selection Change Command is triggered when you select something in a 3D viewport. The command isn’t triggered when you select an object in the explorer.