A ghost portal is a gateway between Earth and the Ghost Zone. People, animals, and objects are able to enter the Ghost Zone and ghosts and/or ghostly objects are able to enter the mortal realm, respectively. Some more powerful ghost beings are capable of creating portals at will.
Within the original Danny Phantom series, there are many forms of portals that exist -- both natural or artificial. For the full list, see the official Wiki page here.
The first one introduced (and most frequently seen throughout the show) is the Fenton Ghost Portal, created and operated mostly by the Fentons in their laboratory. It has undergone destruction and renovations multiple times with upgrades ranging from a simple metal-plated barricade to thumbprint or password accessibility. The Ecto-Filtrator must be replaced every six months or else the portal will overload and create a massive explosion, of which Jack Fenton has a simulation on his laptop to remind himself (and by extension, his family) of the importance in said task.
The second most-frequently seen portal belongs to Vlad Masters, which was originally built in his mansion in Wisconsin. When he forgot to change the Ecto-Filtrator, he lost both his house and the portal, so while waiting for his house to be rebuilt, he was able to use the Fentons' portal when Jack offered their home as a temporary settlement. Before being interfered by Danny, Vlad had almost completely dismantled the Fenton portal, hoping to steal it.
Besides artificial portals, there are natural portals that exist that can lead to random locations and/or time periods, and each one may only exist for an unpredictable amount of time. If humans travel through these portals and fail to return before it closes, they may be lost forever from their natural time and place. Some portals occur in the same or similar places on Earth (such as the Bermuda Triangle, which is infamously mysterious).
Within the Spectral Connections series, the artificial ghost portals owned and operated by both the Fentons and Vlad Masters are relatively identical to mainstream media. Both require replacing the Ecto-Filtrator every six months, possess barricades, require thumbprint identification, and an unholy amount of ghost activity from both realms to which it connects. Pending more...