The Far Lands were the "edge" of the seemingly infinite Minecraft world until thew were removed in Minecraft Beta 1.8. The Far Lands start at 12.550.820 blocks away from spawn. (X (Length) and Z (Width) but not Y (Height) ) This is why Kurt is still playing in version 1.7.3 in his Far Lands or Bust series.
At 12.550.820 blocks away from spawn, terrain generators stop functioning properly. This results in a massive wall up to the Y (Height) limit. At the corners of the Far Lands, (X (Length) and Z (Width) both equal 12.550.820) layers of terrain generate on top of each other.
At 32.000.000 blocks away from spawn, physics stop functioning correctly. Lightning does no longer work and "fake" chunks start appearing. When stepping on one of these chunks, the player will fall trough, straight into the emptiness of the Void.
At 2.147.483.648 the world renderer no longer works, or takes incredibly long to work.
At 34.359.738.368 this gets even worse.
- The Far Lands were removed unintentionally when the terrain generator was updated.
- The Far Lands already existed in the Infdev (In Further Development) version, but they didn't have many side effects and there was no lag.