Final Fantasy 12 introduces a new type of battle system: ADB (Active Dimension Battle). This type of system allows your party to see monsters, and indeed for monsters to see you! Be careful.
Think of monsters in Final Fantasy 12 as similar to those found in Final Fantasy 11. In 11, not only can you see monsters, but monsters can see you and indeed attack when they do. Monsters in this game, for lack of a better phrase, have a mind of their own.
Monsters have the ability to see, hear, and smell you as you approach, just as you have the ability to see monsters from around the corner. When you run across one, a battle begins. No loading screen, no special swirl, but immediate actions.
In battle, monsters also have their own mindset. Some will run away when they see you, while others will call in their buddies to lend a hand. Some may even fight their buddies! There are a number of different options available during combat.