Metabolism encompasses all chemical reactions in the body that convert food into energy and build or break down molecules.

Two key metabolic processes:

• Catabolism — breaking down molecules to release energy (e.g., burning glucose)
• Anabolism — building complex molecules from simpler ones (e.g., building muscle)

Metabolic rate factors:

• Body size and composition (more muscle = higher BMR)
• Age (decreases ~2% per decade after 30)
• Sex (men typically higher)
• Thyroid function
• Genetics

Myth busted: "Slow metabolism" is rarely the cause of obesity. A 200-pound person has a higher BMR than a 120-pound person. Most metabolic differences between people are small (200–300 kcal/day).

Reference:

Wikipedia: Metabolism

https://en.wikipedia.org/wiki/Metabolism