Excerpt from Understanding Pac-Man Ghost Behavior by Chad Birch on GameInternals:
Each of the ghosts is programmed with an individual “personality”, a different algorithm it uses to determine its method of moving through the maze. Understanding how each ghost behaves is extremely important to be able to effectively avoid them...
The key to understanding ghost behavior is the concept of a target tile. The large majority of the time, each ghost has a specific tile that it is trying to reach, and its behavior revolves around trying to get to that tile from its current one. All of the ghosts use identical methods to travel towards their targets, but the different ghost personalities come about due to the individual way each ghost has of selecting its target tile. Note that there are no restrictions that a target tile must actually be possible to reach, they can (and often are) located on an inaccessible tile, and many of the common ghost behaviors are a direct result of this possibility... The next step is understanding exactly how the ghosts attempt to reach their target tiles. The ghosts’ AI is very simple and short-sighted, which makes the complex behavior of the ghosts even more impressive. Ghosts only ever plan one step into the future as they move about the maze. Whenever a ghost enters a new tile, it looks ahead to the next tile that it will reach, and makes a decision about which direction it will turn when it gets there. These decisions have one very important restriction, which is that ghosts may never choose to reverse their direction of travel. That is, a ghost cannot enter a tile from the left side and then decide to reverse direction and move back to the left. The implication of this restriction is that whenever a ghost enters a tile with only two exits, it will always continue in the same direction.
However, there is one exception to this rule, which is that whenever ghosts change from Chase or Scatter to any other mode, they are forced to reverse direction as soon as they enter the next tile. This forced instruction will overwrite whatever decision the ghosts had previously made about the direction to move when they reach that tile. This effectively acts as a notifier to the player that the ghosts have changed modes..