The Dr. Mario series is a series within the Mario series that mainly features puzzle solving. Because of this, this series a spin-off of the main Mario series.
About the series
The Dr. Mario series, as stated above, is mainly a puzzle series that focuses on taking down creatures known as Viruses using mega vitamins. The main protagonist of the series is Dr. Mario who is an alter-ego of the series main hero, Mario. Dr. Mario is usually seen on the top, left hand of the screen passing out mega vitamins onto the screen.
The main objective in every Dr. Mario game is to get rid of the Viruses by matching the correct color mega vitamin onto it (referencing Tetris). The mega vitamins must line up three ways in order to take out a certain color virus. This must be done many times to get rid of all of the viruses and complete the level.