In the Pokemon series, Pokemon can be seen evolving in many different ways. When Pokemon evolve, their shape changes, including their stats too. Not only that, evolving can make them learn different moves too. They can also have different abilities too when they evolve and new typing.
In the Pokemon games, when a Pokemon is evolving, you will be seeing a different screen, with the Pokemon evolving. If you press B during the Evolution, the Pokemon stops evolving. In Pokemon Tower Defense, however, there's a "Stop Evolving" button when the Pokemon is evolving.
To prevent Pokemon from evolving, they can hold an item called the Everstone. In PTD2 you can buy the Everstone for 10 Daily Coins or 1 SnD Coin.
There are many ways to evolve Pokemon. The basic way is by reaching a certain level. Other ways are Evolution Stones, Trading, Friendship, and much more.