The Portal is a block added in the 2.0 Update. Portals allow you to travel from one place to another fast. If you place a portal in one place and another in a second place with the same color, they will link.
If you look closely at the side of any portal, you can see that the texture (excluding the portal part) is just 2 stone blocks stacked on top of each other.
Creatures can go through portals.
Portals are 2 blocks tall and 1 block wide.
If you hold the screen before placing a portal, the placing icon is that of a normal stone block, rather than a portal
Portals originally couldn't go through loading screens and still can't on some devices.