What do you mean "outgas"? Are they losing mass significantly?
Outgassing occurs when a body with components sufficiently volitile as to vaporize when subjected to a heat source (like the sun), has parts of its form vaporize. This can even form actual jets of vapor, if the volitile components have less volitile materials around them.
Typically comets with elliptical orbits will outgas as the come closer to the sun (predominently from the side facing the sun at the time), and the components ejected will either drift off (forming debris trails), or will recondense or recollect on the comets surface when it heads out away from the sun.
The total mass ejected need not be much to move the comets orbit by as little as the diameter of the comet, and can easily alter it by a few miles out of a several-hundred million mile orbital path.