How to Teleport Players When They Enter a Certain Area/Coordinates

I am trying to teleport a player to certain coordinates once they enter an area.

The idea is that they walk into a really small building, but once they turn a corner, they get teleported into a larger building.

Please refer to the adventure map named '5 Cubed', if you need help understanding what I am asking.

1.9 Solution

Place the following command in a repeating command block set to always active:

/execute @a[x=a,y=b,z=c,r=d] ~ ~ ~ tp @p e f g

Where:

a, b, and c are the XYZ co-ordinates that you want to teleport players from
d is the radius in blocks around that point where players will be selected (min 1)
e, f, and g are the XYZ co-ordinates that you want to teleport players to (i.e. the destination)

Use the same command as above, but since no repeating command blocks exist, place the command block on a fill clock.