Aristotle's views of reality were enshrined in what he referred to as his "first philosophy," or what we today call metaphysics. This involved in-depth study of the universal principles and qualities of all material existence. He had studied biological and other scientific phenomenon while employed as a teacher and came to believe that the natural world as we know it was real, physical and tangible. He believed we could use our senses of sight, sound, smell, taste and touch to identify Earth's natural forms. If one could identify something's form, one could explain both what and why it is.

This was a vast departure from the teachings of his mentor, Plato, who, in his book The Republic, used his "Allegory of the Cave" to essentially state that everything we perceive in this world is an illusion—just shadows dancing on a wall, an artificial replica of the real thing, which exists in perfect form in some other world. Aristotle's view that reality is definable and identifiable and tangible as we experience it eschewed Plato's notions of reality as abstract and grounded it in root causes. In other words, if we could explain how and why something was, what it's purpose and uses were, then we could explain what it was.

One problem with trying to understand Aristotle's theory of "reality" is that the modern term does not necessarily correspond to any specific concept or word found in Aristotle. In fact, the popular sense of reality is vague enough that it can be somewhat of an obstacle to philosophical discourse unless explicitlt defined.

Two possible Aristotelian concepts which might address your question are "physis" (nature) and "ta onta" (beings, things that are). For Aristotle, as mentioned by Redscar, Metaphysics was the study of being qua being. The difficulty of this study is that for Aristotle, universals are not separated (there are no "forms") but rather for an substance to exist it must have qualities -- the only pure being is the unmoved mover. The nature, or physis, of all beings can be understood in terms of four causes, the material cause (the substance of an object), the final cause (its purpose), the formal cause (its form or nature) and the efficient cause (what made it)