Metabolism is the general label for all the chemical reactions within an organism that enable the organism to obtain energy by breaking down complex molecules or to build molecules by using energy. These two very different types of results make up the two basic types of metabolic reactions: catabolic and anabolic.
A catabolic reaction involves an organism breaking down a complex molecule to obtain energy for use as needed by the organism. Digestion is the easiest example of a catabolic reaction. A nutrient is taken in and broken down through the interaction of the nutrient and the digestive enzymes. As the molecule is reduced in size and structure, energy is released which can then be distributed and used by the organism as needed.
An anabolic reaction involves using energy to combine smaller molecules into larger, more complex ones. When the body builds muscle or bone mass or repairs itself after an injury, it is carrying out anabolic metabolism to produce the new cells needed.
I found a nice introduction to weight loss where its all discussed:
Whether you are trying to lose 5 pounds or more than 50, the same principles determine how much weight you lose and how fast your weight loss will occur. Remembering the following simple guidelines and putting them into practice can lead to weight loss without the aid of any special diet plans, books, or medications.
Our body weight is determined by the amount of energy that we take in as food and the amount of energy we expend in the activities of our day.
I hope it helped.
And you can also increase your metabolism via activity:
Activity. Get more physical activity and exercise. This is especially important to prevent regaining the weight you lose. Aerobic activity helps you burn calories, and strengthening exercises can help you maintain and build muscle mass.