A deed is an act that is done for the sake of good. It is a selfless act that does not have any kind of expectation in return.
A deed can be anything from donating money to a charity, to giving someone your coat when they are cold. Good deeds should not be confused with acts of kindness, which are often more personal and can involve intimate relationships.
Saving a life is a good deed. Good deeds are typically designed to improve the world, making it a better place.