A Portal is a magic gateway linking two places together. It functions like a hole in space, step through and you will come out the other side of the portal.
The most common type of portal is a Town Portal, which is blue in color. Portals created by Portal Shrines in Diablo II are also blue in color. All other portals in the Diablo games are red. Examples include the portals to Tristram and The Secret Cow Level.