Well what is different about these two pokemon versions? The first would probably be that one cartridge is red and the other is blue, and one has Charizard on it and the other Blastoise. You already knew that. Theres not that many things different about the games. They have the same characters, same plot, and the same objective. First when you start the Red version the default name is Ash, and in the Blu version the default name is Gary.
Now to the important stuff. There are some pokemon that can only be caught in certain versions. These pokemon may be common to you but rare to someon with the other version. Each version has 11 pokemon which the other version doesn’t. If you want to catch ‘em all you will have to trade with a friend. There are also some pokemon that can be found in a certain area. For example, you’ll find Horsea and Seadra in the Seafoam Islands in the red version, but in the Blue version you will find Krabby and Kingler. There are more pokemon in certain areas for each version.
Also there are differences at the game corner prize exchange place. There are some pokemon you can only get in the exchange place. Even the prices differ. In the red version Abra is 180P but in the blue version Abra is only 120P. Below is a chart of which pokemon can only be found in the red and blue versions and a chart showing the differences at the game corner exchange place.