Active Stretching: In active stretching, there is no external force being provided. You are actively moving one muscle group to stretch another. Active stretches can be completed without a stretching partner or stretching accessories. If you are performing the stretch with no external force, it is an active stretch.
Passive Stretching: In passive stretching, there is an external force being provided. You are being stretched instead of actively stretching. Passive stretching can be accomplished through partner stretches, using stretching accessories and/or just using gravity to assist your stretch. If the stretch is reliant on an external force, it is a passive stretch.