The memo is taken as a shortened word for memorandum. The content or subject of its content is usually limited. Memos are considered to be more exclusive in nature. They serve as a means of reminder towards an action that should be taken immediately. You can also use a memo to pass on a proposal or an idea.
On the other hand, when we are talking about a circular, it usually contains various topics. Over time, it has been noticed that circular covers general subject matter. Back in the case of a memo, it is not widely distributed or made available. In companies or institutions, it can, sometimes, only be passed on to the involved individual or group.
This is unlike circulars, which are usually distributed widely to the whole populace of a particular group of people. It usually serves as a form of general announcement. Circulars are also used in advertising stuffs sometimes.