Although defect and flaw are similar, there is a slight difference in usage:
When a person has a flaw in their personality, we do not say they have a character defect; instead, we say "character flaw". Likewise, we don't say something that is broken has a "flaw", instead we say it has a "defect".
In other ways, "flaws" are imperfections that aren't necessarily rejected.
EX: Although the diamond had some minor flaws, the woman bought it nonetheless.
Alternatively, "defects" are imperfections that ARE rejected.
EX: Since the watch was defective, the customer returned it to the store.
There is a bit more to it than this, but the above should help in getting to know the difference between the two words. :) GOOD LUCK!