It's useful to distinguish two kinds of event processing in Tcl,
that having to do with stream-oriented input-output
[[some reference here]],
and the events of [Tk]'s WIMP user interaction,
detailed in "[windowing system events]".  Note, thought, that both exploit [the notifier].

It's also important to understand the [Tcl event loop], and the general concepts of [Event programming and why it is relevant to Tcl/Tk programming].