Androids:

#1-7. Faceless failures.

#8. was a kindhearted creature that became Goku's friend, and therefore was a failure.

#9-15. Failures.

#13-15. were very close to being completed.

#16. Very little is known, but Dr. Gero says he's a failure. It might be his personality, but there may be other things wrong, too. (Machine base)

#17. He was too strong, and therefore a failure. (Human base.)

#18. Not as strong as #17, but she didn't listen to Dr. Gero, and was therefore a failure. (Human base.)

#19. Finally, success. But, is killed by Vegeta. (Machine base.)

#20. He made himself into a android. Is killed by #17. (Part machine, part human base.)

#21. (Cell): Dr. Gero wanted to create a android with bio-technology. But, he couldn't do it in the time he had, so he left the job to his computer. Cell was made up of a combination of cells from Goku, Vegeta, Piccolo, Freeza, and Freeza's father. From this body, if he merges with #17 and #18, he would become perfect.