Non-periodic comets are seen only once. They are usually on near-parabolic orbits that will not return to the vicinity of the Sun for thousands of years, if ever.
Periodic comets usually have elongated elliptical orbits, and usually return to the vicinity of the Sun after a number of decades.
The official names of non-periodic comets begin with a "C"; the names of periodic comets begin with "P" or a number followed by "P". Comets that have been lost or disappeared have names with a "D".