Sooner or later, it finds its way downstream. Throwing it into the trash has the same effect as pouring it out. The oil ends up in a landfill, where it eventually seeps into the ground. How you should get rid of used oil depends on where you live. Used motor oil can be recycled, either as fuel oil or as motor oil again.
Most states have recycling programs to encourage proper disposal.
Simply put the old oil in a leak-proof container, such as plastic gallon jugs or specifically designed reusable ones available from auto supply stores, and take it to a designated collection center. Be sure the oil is not contaminated with anti-freeze or other fluids. If none of these choices are available in your area, there is still help.