|MadSci Network: Physics|
Here's a good page on virtual particles.
Any individual particle can be *either* virtual or real. Some photons are real, and some are virtual. Similarly, some gravitons are real, and some are virtual.
We experience virtual gravitons all the time -- these are the gravitons that our bodies exchange with the Earth, which allow us to feel gravity. The Earth and Sun exchange virtual gravitons; that's why they orbit each other. Black holes use virutal gravitons to attract other matter.
A *real* graviton, on the other hand, has never been detected. Very expensive detectors are now being constructed all over the world, in an effort to see real gravitons. Two orbiting pulsars would lose their energy to *real* gravitons -- if we were close enough to the pulsars, and had a sensitive-enough detector, we could see those gravitons. No one doubts that real gravitons exist -- indeed, the pulsar data seems very convincing -- but we haven't yet seen them.
As for the difference between real and virtual particles... see the above link.
Try the links in the MadSci Library for more information on Physics.