Virtual particles are particles that violate the law of energy conservation .

In particle physics, the law of energy conservation is obeyed in the long term like everywhere else. However, particles can "borrow" a small amount of energy if they give it back fast enough. Like that, a particle which has no energy can borrow energy and have real properties, but only for an extremly short while. When it gives the energy back, there is the same amount of energy as there was to begin with, so in the long term, energy was conserved.

Some particles are partially virtual. This is the case when they have some energy to begin with, but not enough to exist, so they borrow additional energy.

About virtual particles in German. (Virtuelle Teilchen)