Dr. Gero's method of converting a human into an android halts their aging and makes them immune to natural death, effectively making them immortal.
The first androids built by Dr. Gero were Androids 1-7. These androids were completely artificial. However, Gero was unable to control them because of their personalities so he scrapped them. He also destroyed Androids 9-12 for the same reason.
Dr. Gero later developed three more Androids: 13, 14 and 15. However, they all suffered from problems as well and thus, Dr. Gero never used them. However, after his death, Gero's supercomputer was able to complete them and sent them to kill Goku. They ultimately failed in their mission and were destroyed by Goku and his friends.
Dr. Gero later created an android modeled after his dead son, a high-ranking Red Ribbon soldier who was shot by an enemy bullet. This android was dubbed Android 16 and appears to be the largest creations. Gero made him powerful and programmed him to kill Goku, but also gave him a gentle personality to prevent him from being destroyed in battle.
Dr. Gero also abducted two runaways and experimented on them until they became Android 17 and Android 18. However, they were kept in stasis due to their rebellious nature. Eventually, they were released by Gero after Android 19 was destroyed.
The first successful Android developed by Dr. Gero was Android 19, an android capable of absorbing energy. It was modeled after a doll Dr. Gero once brought home. Gero later built an android body for himself and had 19 place his brain in it. Android 19 ended up being destroyed in their first confrontation with the Z Fighters, causing Dr. Gero to release Android 17 and Android 18. Their first act upon being released was to destroy Gero before freeing Android 16 from stasis and attacking the Z Fighters.
While Androids 17 and 18 were incredibly powerful, Dr. Gero's ultimate creation was Cell, a bio-android created from a combination of cells extracted from the world's most powerful fighters, including Goku, Vegeta, Piccolo and even Frieza and King Cold. Cell's goal is to absorb Android 17 and Android 18 in order to reach his perfect form. While the Cell in the main timeline was destroyed by the Z Fighters before it could reach its completion, another Cell arrived from a future where Android 17 and Android 18 are already dead. Cell successfully assimilated the two androids and achieved his perfect form but ended up being killed by Gohan after he transformed into a Super Saiyan 2.