These sound like terms that I use daily in industrial control systems; so I assume that they transfer over to remote telemetry as well.
Discrete alarms are usually simply a 'high' voltage sent out on an input to indicate some sort of an alarm condition that has been achieved. We frequently send these to a SCADA system or an OMNI-dialer that actually dials out to notify an operator that an alarm condition has been achieved.
Contact closures relate to 'relays', which are extensively used in industrial control systems. PLCs (programmable logic controllers; think of them as finite state computers) frequently control attached devices (pumps and valves in my case) that are really either 'on' or 'off'. These are typically turned on with a relay. Basically, the relay is made up of two parts. A coil and a contact. The coil has a charge sent through it which causes the contact to close. When the contact is closed, it closes the circuit allowing the loop power to flow to the device.
So, an external entity can provide 120VAC (or some other form of electrical flow) to the contact and the circuit will only be closed, and the attached device subsequently powered, when a control system sends a charge through the coil.
A digital input really is typically a 5V signal that indicates a boolean true value. The absence being a false value.
As far as organizations go, we have a couple engineers on our team who are members of the ISA - which might be a good place to start. The PLCs that we use are manufactured by Allen-Bradley; popular competitors include Seimens and Koyo - which might also provide some jumping boards for further research on your part.