Portal may refer to:
System Portals[]
System Portals are a collection of blocks that are able to transport Players into system-generated worlds, such as the Cave, Nether and The End. They can be found commonly around their respective biomes in Spawn, other system-generated worlds or any default world.
i.e. Ocean monument portals generate in rivers
The End City Portal and Stronghold portal is unbreakable. However, it may be obtained through Lottery Cases or Christmas Presents.
World Portals[]
World Portals are portals spawned by the "drop portal" option in the world interface. They are always unbreakable, and when placed by someone other than the owner of a world, it disappears after some time. Therefore, only World Portals placed by the owner of the world it is placed in do not disappear.
Portal(Structure)[]
Some System Portals can be obtained by building a structure that spawns it. This includes the Nether Portal and End Portal.
By placing Obsidian in a frame-like structure on the ground, and lighting it with Flint and Steel, Nether Portal blocks will spawn in the frame. They are breakable, hence it is possible to farm them.
By placing End Portal Frames in a frame-like structure, or finding an already placed End Portal Frame in the correct arrangement, then filling the frames with Eyes of Ender, the End Portal is activated and End Portal blocks will spawn in the frame.