Preload is the stretch. The amount of volume being returned to the right side of the heart from systemic circulation. Afterload is the squeeze. The amount of resistance the left side of the heart has to overcome in order to eject blood.
For those who don't remember Latin roots, here are some common medical prefixes and postfixes. Designed for doctors/nurses in training, but since many of us have to make our own medical decisions, it could be useful for anyone.