My purpose is to try to understand what is a video game and what isnt.
There isn't one universally accepted definition of what constitutes "videogame" and what doesn't. Here's my take on it.
First, let's separate the definition of "game" from "videogame". Yes, really.
A "game" is an activity with one or more objectively-determined victory conditions, whereby one or more players attempt to achieve these conditions with their actions governed by a series of objective rules. This covers table-top games, most sports (though the objectivity of referee rulings can be in question), and most things we call videogames.
This definition arises from comments made by Will Wright, about the differences between a "toy" and a "game". Games have goals; toys don't. You can do various stuff with a toy, and you may find that stuff fun. But the toy doesn't say whether you win or lose; that's up to you to decide. It makes no value judgements about how well you used the toy.
Games do have goals. Games do have built-in victory conditions. Games put values on certain actions above others; those that come closer to achieving the victory conditions are better than those that don't.
A "videogame" is a computer program of some form designed specifically for the entertainment of the user(s).
That last part is the key for me. You can have fun with Photoshop and you can have fun with Mario Paint. But these are two very different applications. The former has a clearly defined purpose: to create and edit images. If you find using Photoshop fun, it is only because you like creating and editing images.
Mario Paint is designed to be an enjoyable, fun experience. You poke at it, and exciting things happen. That's why it has a music editor. That's why it doesn't have a lot of the features Photoshop does. So while you can certainly do more with Photoshop, the experience of using Mario Paint is more geared towards user pleasure. You're not creating anything you possibly could or something specific; you're creating fun pictures of stuff. Not particular stuff, but stuff in general. It's a restricted environment, and the restrictions speak to a certain kind of image.
The same goes for web browsers. A web browser itself is not a videogame, even if all you use it for is personal enjoyment. That is not its designed purpose; it's purpose is to feed you information. If you find that information enjoyable, so be it. But it could just as easily feed you Goatse.
You may notice that the definition of "game" and "videogame" have little in common. Mario Paint is very much a videogame, but the lack of explicit goals (built into the rule system of the program) mean that it is not a game.
It's important to make that distinction. Just as you can have games that aren't videogames, you can have videogames that aren't games. The term "videogame" only originated because the early videogames all were games. Space War, Pong, Pac-Man, Defender, all of these had rule-bound action and objective victory conditions (score the most points, hit the other guy, etc).