Short answers, pulled from the story.
Positive feedback is a process in which the outcome of a system reinforces the same process that produced it, causing a buildup of momentum. Mathematically it is defined as a positive loop gain around a closed loop of cause and effect. Both positive and negative feedback play important roles in biology, chemistry, electronics, and cybernetics.
The Larsen effect, named after Danish scientist Soren Absalon Larsen, is the positive feedback loop that occurs when a microphone picks up the amplified output of a loudspeaker in the same circuit, re-amplifies it, and repeats the cycle. The result is the loud squeal or screech familiar from PA systems. The frequency of the sound is determined by resonance characteristics of the microphone, amplifier, loudspeaker, and the acoustics of the room.
"I Feel Fine" by the Beatles is one of the earliest examples of intentional audio feedback used as a recording effect in popular music. It opens with a single percussive feedback note from the A string on John Lennon's guitar. Lennon said in one of his last interviews, "I defy anybody to find a record, unless it's some old blues record in 1922, that uses feedback that way."
The Ferguson reflex is the positive feedback loop that drives uterine contractions during childbirth. A contraction causes oxytocin release, which stimulates the hypothalamus to produce more oxytocin, which increases the amplitude and frequency of contractions. The loop ends when the baby is delivered and the original trigger is removed.
When the loop gain in an amplifier rises above one, the system becomes unstable and can exhibit exponential growth, increasing oscillations, or chaotic behavior. This is described by the Barkhausen stability criterion. Low-frequency parasitic oscillations of this kind are sometimes called motorboating because of their resemblance to the sound of a slow-revving exhaust.
The main positive feedback in global warming is the water vapor loop: rising temperatures increase atmospheric water vapor, which is itself a greenhouse gas and causes further warming. Ice-albedo feedback is a second mechanism, in which melting ice reduces surface reflectivity, causing additional heat absorption. The IPCC Fourth Assessment Report noted that anthropogenic warming could lead to abrupt or irreversible effects depending on the rate and magnitude of change.