What is the difference between iPad and iPad air

You can compare models here:

iPad - Compare Models - Apple

The Air would certainly perform better than the base iPad, but it's significantly more expensive. It really depends on your budget and what use you think your 12-year-old will put it to. As Nick suggested, for basic entertainment and communicating with friends, the base iPad would probably be sufficient. It's quite a capable device in its own right.

If you're in the US, you could also consider a refurbished iPad Air of the previous generation. One of those would be more capable than the current base iPad while still offering better performance, while being less expensive than the current model iPad Air, though the models Apple currently have available are a bit small in terms of storage. See:

Refurbished iPad Deals - Apple

Regards.

iPad Air is a more professional iPad. It features a better chipset and the screen is a tad bigger

For a 12 year old using it only for entertainment options the regular iPad is enough. If your 12 year old also wants to do some photo or video editing the air might become interesting