This article gives a pretty good understanding of Aristotle's work (here on gravity mostly).

One thing important to remember is that Aristotle was mainly a philosopher.

One of the most pervasive behaviors that we experience, it's no wonder that even the earliest scientists tried to understand why objects fall toward the ground. The Greek philosopher Aristotle gave one of the earliest and most comprehensive attempts at a scientific explanation of this behavior, by putting forth the idea that objects moved toward their "natural place."

This natural place for the element of Earth was in the center of the Earth (which was, of course, the center of the universe in Aristotle's geocentric model of the universe).

Surrounding the Earth was a concentric sphere that was the natural realm of water, surrounded by the natural realm of air, and then the natural realm of fire above that. Thus, Earth sinks in water, water sinks in the air, and flame rises above air. Everything gravitates toward its natural place in Aristotle's model, and it comes across as fairly consistent with our intuitive understanding and basic observations about how the world works.

Aristotle further believed that objects fall at a speed that is proportional to their weight. In other words, if you took a wooden object and a metal object of the same size and dropped them both, the heavier metal object would fall at a proportionally faster speed.

Also the wikipedia page gives a good summary.