Pop-A-Tronic is your basic puzzle game that has you removing like-colored bubbles. In order for a series of bubbles to go away, they have to be in a group of three or more. If you click on a bubble that belongs to a smaller group, that bubble becomes "locked" and cannot be removed unless a bomb goes off or it reaches the bottom row.
Marathon Mode is the only one open when you first start the game. Here your goal is to simply last as long as possible. This mode starts off with a tutorial that lays out the basic gameplay rules of the game and is just the beginning of the rest of the game.
Once you have made it into the top 5 High Scores in Marathon Mode several times, Puzzle Mode gets unlocked and here you will have to use your wits instead of just your quick reflexes to clear out the board.
Lock-Out Mode is the next available mode (again obtained after making it into the high scores of Puzzle Mode) and here you have to not only race against the falling bubbles, but you also have to contend with an ever-shrinking gameboard.
The final mode is Self-Destruct Mode and might not feel very different from Marathon mode, but this time you have to worry about explosives and a counting down timer.