The Problem of Universals
Universals is another name for the Platonic Ideas or Forms. Plato thought these ideas pre-existed the things in the world to which they correspond. For example, a perfect circle is an idea to which any actual circle would only correspond approximately. Aristotle thought the universals were merely general versions of properties found in common in the particular things. Thus the general idea of a horse would be the bundle of common properties that are found in all particular horses.
The terminology is a bit confusing because Plato regarded these Ideas as "real" where today we regard the things as real and the universals as ideal. The great problem of the universals has been "do they exist?"
Both Plato and Aristotle have anticipated the Information Philosophy view of Universals. The Universal is simply the information which is a limited subset of the common information found in all the particulars.
The great ontological and existential problem of the universals then becomes clear in Information Philosophy. The tangible existence of an idea depends on it being encoded somewhere, in a mind, in a physical or biological structure, etc. The information encoded might be a mathematical concept, for example a perfect circle as all the points equidistant from a reference point.
The mental abstraction of a universal away from any and all minds remains an issue with two resolutions. First, the universal idea has probably been encoded in human artifacts - books for example - now independent of any particular mind. This is our Sum of all information. Second, with Aristotle, we can imagine that many particulars pre-existed any minds and would remain in the world in the absence of any mind, containing their generals for any future intelligence to discover.