A portal, or rift, is doorway that exists between different realms, planes of existence, or universes. They are usually hidden and often guarded with magic and rituals.
It's a tear in space and time... a doorway to another world.
As Jack Kline the Nephilim is close to being birthed, his power surges through his mother and is released, causing a puncture in the fabric of the universe and opening a door to Apocalypse World, an alternate version of Earth. Castiel, Sam, and Dean travel back and forth through it, ultimately using it to trap Lucifer on the other side. The portal is closed due in part to Crowley sacrificing himself as the final ingredient for the ritual to seal the rift. The portal fully closes as Jack is born, just as Lucifer is knocked back into it and pulls Mary through to Apocalypse World with him.
A second portal between the Earths was created with the aid of the alternate universe version of Kevin Tran. Through deciphering the angel tablet, Kevin was able to create a spell to open a new rift using a portion of Lucifer's archangel grace. The spell, however, was designed to admit one person at a time and closed after Lucifer made his escape through it. He landed somewhere in Cincinnati, Ohio.
The Winchesters later learn of a spell from the demon tablet that can open a portal to alternate universes; it requires the blood of a most holy man, fruit from the Tree of Life, the Seal of Solomon, archangel grace, and something that has already been to the world in question to act as a guide. After gathering all of the ingredients, Sam is able to use the spell to open a twenty-four hour portal identical to the one opened by Jack's birth which is used by Dean and Ketch. It later closes after the twenty-four hours are over.
After convincing Gabriel to help, the Winchesters, Castiel, and Rowena make a failed attempt to open a portal using a small amount of Gabriel's grace. The portal opens but quickly fizzles out. In order to get the portal open and keep it open, they capture Lucifer and drain his grace continuously in order to keep the portal open indefinitely. After Lucifer is accidentally flung through the portal by Rowena, he leaves behind enough grace to keep it open for another thirty-one hours with Rowena using power from the Black Grimoire to extend that time a little to enable a mass evacuation of dozens of people from Apocalypse World. Once everyone is through, the portal closes and Rowena tells Sam that she couldn't have held it open any longer. Lucifer and Michael later use the same spell to open a portal of their own to reach the Winchesters' world.
Fandom nicknamed the portal the "glowy space vagina."
While trying to create a portal to Apocalypse World, Jack Kline and Kaia Nieves accidentally create a portal to the alternate universe known as the Bad Place by mistake due to Kaia flashing between the Bad Place and Apocalypse World while dreamwalking for Jack. The portal is identical to the one for Apocalypse World and remains open for a few days before closing on its own.
Dark Kaia is able to open an identical portal from the Bad Place into a Sioux Falls park and come through. She later explains to Dean and Castiel that she opened her rift using magic originating in the Bad Place. However, that same magic doesn't work in the Winchesters' world and she can't use it to open a portal home. Instead, Dark Kaia requests the help of Jack to open a new rift for her to go back to her home.
After Metatron expelled all the angels and locked the Gates of Heaven, he created a piece of spell work that allowed him to come and go from Heaven to Earth. The spell simply requires a specific sigil -- Metatron's Cube -- to be drawn. Once done, it creates a doorway into Heaven.
The portal is currently located in a playground. Prior to Metatron's reign coming to end he was known to move the location of the entrance around. It is always guarded by one or two angels, who draw the sigil in a sandbox. As Metatron's spell has not been reversed, even after his capture, this became the only method by which angels can access Heaven from Earth. When the Cosmic Entity invaded Heaven to take Jack's soul to the Empty, he opened all the gates sealed by Metatron. However, the angels continue to use the portal in the playground as they still have no wings.
In the Abrahamic religions, the idea of a "stairway to Heaven" is known as Jacob's ladder.
There are a number of Hell Gates, also known as Devil's Gates, which are portals between Hell and Earth. When opened they allow demons passage to Earth. One of the major gates is the Devil's Gate in Wyoming which Samuel Colt built a huge devil's trap around to prevent demons accessing it. There was a church at each point and the sides of the pentacle were made of iron railroad lines. Azazel plan for Lucifer's release included opening the gate to release demons to use in a war against humanity.
As the devil's trap prevents him from getting to the Gate, he uses Jake Talley -- one of the Special Children to cross into it and unlock the gate using the Colt as the key. When the door is open the blast of energy lights up the surrounding railways and blows up at least one of the churches, which significantly weakens the devil's trap and allows Azazel to enter. Hundreds of beings, including demons and spirits, escape from Hell before Bobby and Ellen manage to close the gate. John Winchester is among those to escape through the gate.
There is also a secret "backdoor" entrance to Hell located in Purgatory that is used by rogue reapers as a means of ferrying souls around. The location of the portal can be found by following a specific stream to where three trees meet as one. Where they meet, there are rocks, and between the rocks is the portal.
The second ritual was performed by dragons as means to bring Eve, the Mother of All Monsters, to Earth. This ritual came from an ancient manuscript; it requires a virgin sacrifice to serve as an Earthly vessel for Eve, the blood of the summoner, and an incantation in Latin. Once the first line of the incantation is spoken, a doorway to Purgatory will open, at which point the blood of the summoner must be bled into it, and the virgin sacrifice thrown into the portal.
The third ritual was acquired by Castiel and Crowley through the torture of Eleanor Visyak. This ritual requires a special sigil painted in the blood of a virgin, and a native of Purgatory. The final part of the ritual requires a Latin incantation to be recited on the night of a lunar eclipse.
As there are ways into to Purgatory, there is also a way out designed specifically for a human, if one was to ever end up there. The portal is located atop a cliff; once a human steps through, they will be transported to the 100-Mile Wilderness in Maine.
French Mistake World
Balthazar appears gives Sam and Dean a key to an alternate universe where he has stored the stolen angelic weapons of Heaven, which will help Castiel in his fight against Raphael. However, this was revealed to be a ruse. Sam and Dean find a world where their lives are the basis of a TV show, and magic and the supernatural don't exist. The portal is a window in Bobby's house on one side, and a window in the TV set of Bobby's house on the other.
Access to Oz can be achieved through the use of one of the six keys forged in Oz. If the key is used on any door, said door will turn into a doorway to Oz. Conversely if the key is used in Oz, it will become a doorway to Earth. Entrance to Oz can also be achieved through natural disasters like tornadoes, whirlpools, or the eye of a hurricane.
The Wizard of Oz was also shown to be powerful enough to create a portal from Oz to Earth through the use of magic, using a mirror as a conduit.
Tentacle Porn Land
Dubbed "tentacle porn land" by Dean Winchester, this alternate universe is home to the cosmic deities Yokoth and Glythur, who have consumed and destroyed most of it. A rift between the universes is created by a rogue sect of the Men of Letters led by Diego Avila using a spell that requires the blood of a most holy man, fruit from the Tree of Life, archangel grace, and the Seal of Solomon.
Nick performs a ritual designed to resurrect Lucifer from the Empty. When completed, a black, viscous portal to the Empty opens, allowing Lucifer to cross over back to Earth. Before Lucifer can possess Nick once more, Jack arrives and sends Lucifer back to the Empty, closing the portal as well.