Universals
Universals or properties are posited by philosophers to explain the similarity of numerically different things. When we say, for instance, that there are "two people driving the same car", it is common sense for most people that they are driving two different cars but those cars have shared properties - they are made by the same company, are the same color, have the same engine and so on. In the language of this branch of metaphysics, the two cars are particulars, but those particular cars share properties, attributes or universals.
In discussion of universals, properties are usually distinguished from relations: a person may drive a red car, but they may be three inches taller than their brother. The former is a property, but the latter is a relation.
How do philosophers understand universals? Quite simply, the two main opinions held are those of realism and nominalism. Realists hold that universals exist in some sense - that a red bus and a red book both have the property of red, and that this property is the same in both. Nominalists, on the other hand, deny this - stating instead that there are similarities between things, but these are not anything more than 'tropes'.
The first philosopher to discuss properties that we know of is Plato in the Parmenides dialogue, and throughout the other dialogues as 'the Forms'. One problem with the notion of universals is whether it is possible for a universal to exist even if no particulars hold that universal. According to Plato, the Forms have a non-physical existence - that even if we were to destroy all the bottles in the world, the Form of the bottle would exist both before and after the particular individual bottles. Aristotle rejected the idea of universals existing outside of the particular instances. The Form of a bottle exists inside each individual bottle[1]. Realism has been held by modern philosophers including Bertrand Russell and D. M. Armstrong, while nominalism's twentieth-century advocates have included Willard Van Orman Quine, David Lewis and D.C. Williams.