Frieza: Frieza once was the most powerful fighter in the galaxy, until Goku overthrowed his evil reign. Gohan, Krillan, Piccolo, and Vegeta fought Frieza as long as they could hold out until Goku arrived and took over. When Frieza killed Krillan, It caused Goku to turn Super Sayjin for the first time, and led to his defeat.
Raditz: Raditz is Goku's brother from the planet Vegeta. He came to earth to enlist goku to help the last of the saiyans conquer more planets. When Goku turned him down, he took Gohan back to his ship. Goku and Piccolo teamed up for the first time to defeat him. Although they defeated him, Goku was sent to another dimension.
Nappa: Nappa was Vegeta's partner until Vegeta turned on him, and killed him. He and Vegeta went to Earth to find out about the Dragon Balls. Although Nappa easily defeated everybody except Krillin and Gohan, Goku proved to be too much for him.
Ginyu Force: The Ginyu force was an elite team of fighters that worked for frieza, the five members were:
Guldo: Guldo was "a little green man" with the power to freeze time. Krillin and Gohan had him on the run, when he used his "mind freeze" attack. Right before he killed them, Vegeta stepped in and finished him off.
Recoome: Recoome was the second one up against the Z-fighters. He fought with them, and easily defeated them. Right before he killed them, Goku made it to the planet and defeated him with a one-hit ko.
More coming.......