If you play a lot of free online games, you will notice that a lot of them look and play alike. There are several specific types of games that developers usually copy, adding their own graphics and twists to the games.
For instance, three-in-a-row is a type of game where you are faced with a grid of objects and can swap two that are next to each other. If this creates a series of three in a row, those objects are removed and more objects fall down to fill the gap. This game has been recreated hundreds of times throughout free online game sites.
Perhaps the most common free online game is whack-a-mole, or point-and-click games. The genre is named after early mechanical arcade games where moles would appear through holes in a box, and you would whack them with a club.
Online, there are thousands of games that show a scene and have items or characters suddenly appear and you have to click on them. Sometimes a click signifies a gun shot, sometimes it signifies that you "see" the item.
These games are easy to develop, especially for creators that have the ability to make artwork easily, but may not understand complex programming.