some doubts

I have some doubts of contract.

1) What is different between voidable and invalid?
I think invalid means not based on all the facts, and therefore not correct and voidable have almost same meaning which is capable of being made or adjudged void. I am not really known that what is actually different between them.

Free consent is another essential of a valid contract. Consent means that the parties must have agreed upon the same sense. For a valid contract it is necessary that the consent of parties to the contract must be free. Free consent is said to be missing due to existence of five factors which are fraud, misrepresentation, coercion, undue influence, and mistake. If contracts are entered into by any of these factors, they will become voidable.
2) I am very confused of fraud. Mr. Sonny Zulhuda told us a example of fraud during class, I am very confused with something about delay, something like if you did not do anything the contract will be valid (I am sorry that I cannot remember details of that example), I am very confused with fraud.