The General Assembly is the "Talking Shop" for the members of the UN. General debate takes place there and decisions are made about all sorts of things.
The security council has a much more specific task, and that is the peace and security of the world. For some other tasks, it also acts as "the Executive" for the General Assembly.
The UN was structured like this so that matters requiring speedy action (for instance if you are trying to avoid world war) did not have to go to the big and clumsy General Assembly.