Bloons is a game about a monkey using darts to pop colored balloons called Bloons. This is the first game in the series. The developer, Stephen from Ninja Kiwi, stated that the game "...is based on the very simple fact that popping balloons is fun."
You're given a small monkey with a handful of darts. The game has 50 levels and the goal on each level is to pop as many Bloons as possible by throwing darts at them. To throw, you aim and hold down the mouse button to gauge the power of the throw, then release the mouse button to send the dart flying. To beat the current level and unlock the next level, you have to pop enough Bloons to reach the target percentage of Bloons using a limited number of darts. Hitting special types of Bloons provides special effects to a level, such as tacks shooting out from a Bloon when it's popped, throwing triple darts, ice Bloons, exploding Bloons, and boomerangs. There's even a Pac-Man Bloon that summons Pac-Man and lets you guide it around to consume other Bloons.
There's an Unlimited Darts Mode which allows you to play a level with unlimited darts in order to get 100% of the Bloons on that level. This mode is only intended so the player can experience all the levels available, and does not allow for high scores. Beating a level in this mode does not unlock the next level in regular mode.