Portals allow travel between, or within, the planes. Their appearances, costs and descriptions vary, but they are usually fairly easy to identify in-game as they are mentioned in the "Doors and Portals" portion of the interface. Some portals, though, are visible only to certain alignments.
Characters cannot create a Glyph in the same tile as a portal.
The cost of entering a Portal, and its approximate destination, is unknown to you until you have bought the skill Sense Magic. However, this cost is always the same for the same portal. So an attentive person might remember the cost and not need this skill to identify those values. The locations and costs of the known permanent portals are listed on the Maps provided by the community.
Ferries allow transportation around Valhalla. The names of these mundane portals will usually reveal their destination (e.g. "Take Ferry to Windfall Ridge" rather than "A curtain of swirling colors") to people without Sense Magic, but they use the same mechanics as magical portals in all other ways.
A portal can be "bent" to a character's "home plane", overwriting the existing destination and increasing the portal's cost by 3 AP and 5 MP. A character's "home plane" is whichever plane contains their standing Stronghold, unless the stronghold is in Valhalla. Angels can also bend portals to Elysium, and Demons can bend portals to Stygia. In all of those cases the character will appear on a random portal exit in the target plane. Note that the character will not always appear inside a portal - the character could appear at the exit to a one-way portal, and so could be miles away from any portal.
The option to "bend" a portal only appears if the character has at least 3 AP and sufficient MP to pay the cost of the portal, plus the extra 5 MP. A character cannot go into negative AP by bending a portal.