The Fenton Thermos is an invention from the original series. The Wiki page is located here.
In the original Danny Phantom series, this invention by Jack and Maddie Fenton is first seen in "Mystery Meat," at which point neither one knew how to activate it properly. Unbeknownst to them, Danny obtained the thermos and managed to activate during his battle with the Lunch Lady Ghost by powering it with ectoplasmic energy. Since then, it has become a frequently-used weapon against ghostly enemies. The thermos is capable of housing multiple ghosts at any given time with the sole intention of returning them to the Ghost Zone where they belong. Danny is never seen using it within the Ghost Zone as a result.
It is capable of containing any type of ghost, regardless of shape or size, but if a ghost is powerful enough to resist containment, it may not hold them long enough to return to the Ghost Zone as planned (such as Dan Phantom in "The Ultimate Enemy" or a ghost-version of Freakshow in "Reality Warped"). If it is damaged, it can release any and all ghosts inside. The thermos also has a setting called "Reverse Polarity," which Danny demonstrates is a way to seal up portals leading between Earth and the Ghost Zone.
While only seen once, Vlad Masters gave his former ghost-fighting employee Valerie Gray a red thermos, which was directly based off of the original Fenton Thermos. Unfortunately, Valerie could not figure out how to use it properly (or even open it), so she is never seen using it.
Within the Danny Phantom: Spectral Connections series, the Fenton Thermos is the relatively the same as it was in the original series. Invented by Jack and Maddie Fenton, it is a device always on hand by the members of Team Phantom since following after "Phantom Planet."
Originally, the only people to own and operate one of these objects (besides its creators) was Danny, Sam Manson, Tucker Foley, and then later on, Jazz. In this series, it is also a device used to symbolize the acceptance of a new member to Team Phantom.