From Diablo Wiki
Portal Stones are altar-like objects found in numerous dungeons in Diablo III. These stones allow characters (plus their minions and follower) to instantly teleport to the entrance of the dungeon, thus saving the time of running back through empty corridors. These objects are purely a tool for convenient travel.
When using one of the stones, there is a short (five second) casting time. If a character takes damage or moves or uses an ability during that time, the casting animation is canceled and must be restarted from the beginning.
Portal Stones are only found in fairly large, dead end dungeons. They are not found in small dungeons, and never on levels that have an exit to find, or that descend deeper.
Players may choose to travel back to town directly via the Town Portal, but often the dungeon entrance is more convenient, if a player wishes to return to exploring the area they were in previously.
Portal Stone Locations
A few portal stones are found in the levels present in the Diablo III beta.