Any herbal derivative from the leaves of green teas can be called a green tea extract. Green tea extracts contain various polyphenols such as catechins that are powerful antioxidants.
Green Tea extract as a diet supplement
Green tea extracts are used as a dietary supplement, in pills and capsules, to go with workouts and are supposed to help you burn more fat while you exercise.
In a race where everyone wants to become slimmer and fitter, people will try anything to get ahead. Green tea extract is a popular weight-loss supplement that people can’t get enough of.
Green tea extracts can be effective in shedding weight because it increases the fat oxidation in your body and reduces body fat. 
It is important to note that taking green tea extracts only will not help you lose any weight. The catechins will help you boost metabolism and increase daily energy expenditure, but only if you also follow an exercise regime.
A Penn State research, conducted by Joshua Lambert, concluded that mice that were given green tea extracts and followed an exercise regimen showed significant reduction in body weight than compared to mice that were not given the extract. 
Green tea extract promotes health
Green tea extract, which is rich in EGCG, has twice the antioxidant property than Vitamin C. Because of its high antioxidant potential, green tea extract may be quite beneficial to your overall health.
The catechins scavenge the free radicals in your body and can prevent several health problems such as obesity, diabetes, high blood pressure, gastrointestinal disorders, certain types of cancer, gum diseases etc.
Extracts high in catechins, were found to reduce the level of LDL cholesterol, the bad cholesterol, which suggest that they can help in preventing cardiovascular diseases. 
Research on the effects of green tea extracts on dental health has revealed that they may be used in rinsing to rid yourself from bad breathe. 
Things to consider before you use green tea extract
Green tea extracts can contain over 50 times more polyphenols than what you would get from a cup of green tea. This means you are getting high doses of EGCG, which is not beneficial. EGCG overdose has been linked to vomiting and liver damage.
Green tea extracts are considered safe if you take them appropriately, but, it has adverse effects if you take them on an empty stomach or in excessive amount.
So, it is advised that you take these extracts moderately.