Wall-In at Natural
Construct a row of buildings at the natural expansion choke to constrict or eliminate access to both the natural and main bases.
Rather than a build order, this is a layout strategy that allows you to wall off your natural expansion, allowing for safer (and earlier) expansion and establishment of two mining bases. With larger maps like Frost and Deadwing FE-builds became very popular in PvZ.
The larger the choke, the more buildings needed to wall, meaning the wall will be slower. You could choose to sacrifice tech or economy, if you want to complete it more rapidly.
Pylons are not optimal as part of a wall-in in PvZ, as these make Baneling Busts more effective.
Make sure to place the buildings carefully. A simple slip up such as placing the first Pylon too far back can be fatal. Make certain the cannon covers all of the buildings from melee attacks. If the map allows it, leave a single space open for a Zealot so no buildings must be destroyed for units to get in and out of the base. If a wall-in building must be destroyed, the Cybernetics Core is usually the best option. The Core must be rebuilt somewhere else, of course, as it is an important tech building.
Maps with wide natural expansions, like Inferno Pools, are not feasible for walling in at the natural. On those maps it is possible to wall to your Nexus.
If you choose to do so, Scouting is very important. You might want to create two cannons early on: one guarding your buildings, and the other protecting your workers. If a Zerg opponent goes for early roaches, it becomes very difficult to hold. The extra cannons will be behind nothing, exposed, thus you must mass a lot more than normal, decreasing the overall economic gain that's supposed to be obtained from walling in at the natural.
Wall-In Examples On Popular Maps
Terraform supports a "YufFE-Wall" and a "normal" wall as well. Like Frost (see below) you can wall off with two buildings. The third has no protection for workers, so if a mineral patch is outmined, Zerglings can flood this mineral line.
On some spawning positions are destructible rocks like on Yeonsu. FFE and YufFE are both possible on Foxtrot Labs. Two buildings and a Cannon are sufficient, to close the wall. If you scout a baneling bust, you should put a Gateway in front of the Cannon. Adding additional Cannons is strongly advised in lower leagues (see below).
King Sejong Station
Unlike any other map you can use your gas to wall with three buildings. A Zealot completes the wall.
Two buildings complete the wall to your Nexus. You can place a cannon on the high ground, if you scout 7RR. Try to protect cannons on the low ground with it.
Three buildings complete the wall to your Nexus. A Pylon is always a target for Banelings (Baneling-Bust). So you should scout and add additional cannons.