Ceramides are natural lipids (fats) that are found in high concentrations within cell membranes. They allow skin to retain moisture by forming a protective barrier.
The reason that we are seeing synthetic ceramides show up in beauty products lately (for example, Elizabeth Arden has an entire line of ceramide-infused skin care products) is because they are able to help replace the fats that get lost in our skin structure as we age.
New evidence leads us to believe that ceramides can also be released by enzymes and act as a signaling molecule. Since common signals are programmed cell death, ceramides are sometimes referred to as "messengers of cell death". They're pretty bad-ass like that.