There are POSIX signals (linux man), and various event-based operating systems (such as TinyOS, most seem to be targeted at embedded systems). But the mechanisms seem very similar.
Are the terms 'signal' and 'event' interchangeable? Or are signals a sub-set of events? Or are they two fundamentally different for some reason that I am completely missing?
As general concepts, the words signal, notification and event are mostly interchangeable, though they carry different connotations and some words wouldn't be used to denote some specific mechanisms. A particular operating system or other concurrency framework may use these terms for different mechanisms.
Some differences to watch for (with signals, events or whatever they're called) include:
For example, POSIX signals are asynchronous, unicast (multicast if sent to a process group), they stack, and have a tag which is a small integer. I'm not familiar with TinyOS; from a quick read of the tutorial I think they are asynchronous, unicast (perhaps with other possibilities), non-stacking, and have a payload (and, incidentally, sending an event is called signalling it; receiption is called triggering, because the event in TinyOS terminology is the handler instance rather than not the message).
I think the word event wouldn't be used if there is no associated information at all. The word signal wouldn't be used if there's a rich payload: with a rich payload, it's a message, possibly an event or a notification. GUI frameworks often call notifications of user input events; GUI events have a payload (e.g. to identify which key was pressed) and don't stack.
The use of the word event tends to promote event-driven architectures, where programs are structured as event callbacks rather than as a thread that checks for notifications periodically, but it isn't limited to this use.
