The Pokémon Browser is like this game's version of the Pokédex, recording data on every Pokémon that you've seen and captured. When you tap a Pokémon with your stylus in the overworld, its name will be added into the Browser, however all data about it (its assist, height, weight, etc.) will be hidden until you capture it. Once you do capture it, its data will be filled out in the Browser.
Pokémon Ranger is a generation 3 game, and has a total of 210 obtainable Pokémon from the first 3 generations.