Aristotle was interested in the material world which he saw about him. He was interested in the nature of things and their substance. However, Aristotle was still interested in questions such as what is it about a table that gives it its tableness? However, unlike his teacher Plato, Aristotle believed that the form of an object was not some kind of abstract ideal. He believed that the form of an object was contained within the object itself. To put it another way, its form was within the structure itself. This meant that the form of an object could be perceived using ones senses.
Aristotle uses the word substance in many ways which often makes it difficult to grasp his concept. Let us look at the example of a table. The substance of a table is the wood and the nails and the glue. However, the form of the table is that it has four legs etc.
To confuse things further, Aristotle also used the word matter to mean the stuff of which something was made. A chairs matter is wood! Its form is the structure of the chair itself i.e. that particular chair NOT some abstract universal.
This allowed Aristotle to also wondered whether it was possible that something could have matter but no form. He concluded that there could be prime matter or stuff that has no particular form and not arranged in any particular structure. Likewise, Aristotle wondered whether something could have form and structure without having matter. He proposed that something that has form and structure without matter is God.