It is the leading cause of death in the USA. Unstable angina, stable angina and myocardial infarction constitute the spectrum of the disease.
Angina is defined as unstable if it fulfils any of the following criteria:
Angina at rest, or angina for the first time, angina after an MI, angina of increasing severity or duration. 
It is said to be stable if it occurs on exertion and disappears shortly after rest (within 5-10 minutes)
Myocardial infarction is defined as muscle damage to the heart.

The most important mechanism of unstable angina is plaque rupture. Risk of plaque rupture increases if there is a thin coat of fibrous tissue over it, or thee is a lot of inflammatory reaction around the plaque or if the core is large and filled with fat.

The single most important intervention in a patient with USA or MI is administration of Aspirin and it is advised to be given right at the onset of the symptoms.

Nitrates do not save lives.