From memory...
SelectWindow was the response to clicking on a window. It brought the window to the front, activated it, and (usually) made it the first responder. (I'm using Cocoa terminology here.) It's sort of like OrderFront
The SelectWindow function removes highlighting from the previously active window, brings the specified window to the front, highlights it, and generates the activate events to deactivate the previously active window and activate the specified window. If the specified window is already active, SelectWindow has no effect. Call SelectWindow when the user presses the mouse button while the cursor is in the content region of an inactive window.
Activate Window updated the window frame to indicate that the window was the first responder.