1. A compound noun is a fixed expression which is made up of more than one word and functions as a noun. They can be written as two words (tin opener, address book, ...), with a hyphen (post-office, ice-cream, ...) or as one word (earring, teapot, ...).

2. Compound nouns may be COUNTABLE (alarm clock, burglar alarm, heart attack, tea-bag, credit card), UNCOUNTABLE (birth control, junk food, pocket money, food poisoning) or only used in either the SINGULAR (arms race, greenhouse effect, sound barrier, death penalty) or the PLURAL (human rights, sunglasses, traffic lights, kitchen scissors).

3. A large number of compound nouns are based on phrasal verbs. Nouns based on phrasal verbs are often informal and are very common in newspaper reporting:

There was a break-out from the local prison (escape)
What the computer produces depends on the input (information that is put in)
Output has increased thanks to new technology (productions)
I can easily get you a printout of the report (paper on which computer information has been printed)
A breakthrough has been made in AIDS research (important discovery)
There are drawbacks as well as advantages (negative aspects)
The outcome wasn't satisfactory (conclusion)
Teachers need feedback from students (comments)
Many of the problems were caused by a breakdown in communications (failure)