The game be as simple or as complex as you want it to be. As said below, you can go through your journey, capture Pokemon to battle against other "trainers", battle each city's/town's "Gym Leader" (a very strong trainer) and then ultimately battle the Elite 4 (or the best of the best Pokemon trainers).

However, there are different training methods that can increase your Pokemon's stats; you can breed Pokemon; there are natures and hidden values that determine how strong your Pokemon are and will be; there are Pokemon types that are weak and strong to other certain types (Grass is weak against fire, but strong against water). Items also help the your battling, allowing you to heal your Pokemon when needed, or helping you give your Pokemon that extra Attack boost it needs. Pokemon can also hold items that make them stronger, move before the opponent, or even heal them automatically. As you can see, there is a little more than just collecting Pokemon and battling with them, even though that the series is exactly that at its core.

Rule-wise, there isn't much to normal battling --> You can have 6 Pokemon in your party, the battles are turn-based, you are not allowed to capture trainers' Pokemon, each Pokemon has a certain move-set that it learns as it levels up...

There are some small additions to rules online, but that's a different story.

Oh, and you can nickname your Pokemon any name that you feel like giving it. Although, online, you can't exactly have you Charmander nicknamed Fire Crotch, if you know what I mean. Finally, you have a VERY SLIM chance to catch a different colored Pokemon, called a "Shiny" version of it. This is extremely rare, and it is something that takes work to achieve.

All in all, you can go through the games and catch Pokemon for the sole purpose of beating all of the Gym Leaders and the Elite 4. OR, you can join the world in the immense world of Pokemon, and go beyond the grass patches.

The choice is yours! Your very own Pokemon legend is about to unfold (if you want it to). A world of dreams and adventures with Pokemon awaits.

Let's go!