How would you describe what's Pokemon to someone who live under a rock?

What can I say more than the title?
I don't remember who said that we are all talking together right now because of imaginary monsters.... but it's kinda true.

Pokemon is a video game series in which you catch monsters and make them fight other monsters.

That's the most basic definition of what Pokemon is.

Welp! Let me make an unnecessarily long reply to a simple question as usual. ;D

Pokémon is the combination of the words Pocket and Monster. It is much like our world, except that you can befriend yourselves with the native creatures that are referred to as Pokémon. ^^

You play as a young protagonist trying to either capture them all, or become the champion of the region your characters lives at, while being provided with one Pokémon to start out with. =) No, you do not go out and hunt them down with weaponry, but instead use a ball, a so called Pokéball (Pocket Monster Ball) which is as large as your hand, yet able to absorb the being into it, despite being many times larger. You could say that it's essence is stored in data, much like how warping seems to work in some of these Sci-Fi movies or tv shows, and that they can be either summoned by the use of said ball or be stored in a computer within the game. ^^ There are many types of balls, some with higher and some with lower capture rate. If the ball has a weak capture rate, the chance to capture the Pokémon is poor, and it will free itself out of it. You may have to purchase a few of them to succeed in capturing said creatures, so make sure to have a lot of so called Pokémon Dollar (or Pokédollar for short).

You can obtain said currency by battling with other trainers with the aid of the creatures you capture, or selling items in the shops which are referred to as PokéMart. Pokédollar seem to work like Yen, in which an item my appear to cost hundreds, when in reality it would only be a couple of U.S. or Canadian dollar. ^^

The more you train your creatures, the stronger they will grow. Occasionally, when reaching a certain threshold throughout their training, or in other words a certain Level, it may evolve. It will change, undergo a rapid age growth or metamorphosis in a matter of seconds, while having a bright light conceal its body throughout the process. They may differ slightly or greatly in their appearance and skills, but mostly benefit your progression through the game.

On a side note, for whatever reason, your character seems to only have a mother (for the most part) but no father. It seems that in the country the game was developed, that the father of the family is no where to be seen due to having to work or are divorced. That detail isn't important to the plot, but it may help to point out. =)

Throughout your journey you likely will have a rival. That may be to encourage you to try harder and keep beating the challenges that he throws at you in the form of a battle.

Unlike real life, the creatures in that world will have abilities that aren't well explained. Some can breathe fire like mythical dragons, while others can use other elements of nature to combat foes. Only few will act like real life beings, like those that are considered Fighting-, Normal-, and Flying-type, only to list a few. Every one of these has their own set of weaknesses that you need to learn about to exploit, or to prepare yourself for them.

In short: Pokémon are monsters that fit in your pocket, able to wield strange powers, and the game is about you trying to work with these creatures and fulfill your task of either capturing them all, beating the championship, or both! ^^

Done! (hopefully)

Okay, I'm on my break! *drinks a jug of water*

I used to explain it to my older brother something like this, Pokemon fight each other, there are different elemental types that means some do more damage to others. When they get strong they 'evolve' which means they turn into a bigger, more powerful Pokemon.
If I have to explain it today as a cynical adult, its a game where kids run freely around the world engaging in simulated dogfights and taking money from the loser.

I'd tell him it's an animal hording simulator with in game animal cock fighting. XD

Rather than saying it's about fighting monsters, I'd say it's about catching and collecting them throughout your journey to make it a bit more child-friendly. At least to me, the collecting aspect is what defines Pokémon to me more than the fighting aspect.

Pokemon is a game where you travel around a region, catching monsters and battling them against other monsters to eventually become the best at the battling of the monsters.