The Namek Dragonballs were the original set of Dragonballs. These balls were about the size of a large bowling ball.
The Earth Dragonballs were a smaller version of the Namek Dragonballs. These balls were about the size of a baseball.
History of the Dragonballs
The First Dragonballs -
Not much is known about the very first dragonballs. The Nameks inherited the Planet Namek after it's birth and used their amazing abilities to create the first set of 7 Dragonballs. The first dragon itself was modeled after the Namek race. It had similar characteristics of the Nameks, but had scales and spikes that protruded out of it's back and limbs. This set of dragonballs started it all.
Tragedy and the second set -
The life of the Nameks and their dragonballs went on as usual. Until Namek started to experience heavy storms and harsh, long periods of drought. Crops died, bodies of water dried up, and the Nameks started to die out. This went on for many years until this cycle left Namek's atmosphere. In the wake of this cataclysm, stood one survivor. The eldest Namek named GURU. Guru began spawning children to re-inhabbit Namek. Life started over on the planet.
Since the eldest Namek who created the first set of dragonballs died in Namek's bad season, a new set of Dragonballs needed to be created. Since every Nameccian has the ability to create dragonballs, Guru took it apon himself to do so. He modeled the second dragon exactly like the first dragon, and named it PORUNGA. This set of dragonballs gave the summoner 3 wishes. This new set stayed with the Nameks until Guru's demise on Earth. This was the last of the "original" Namek Dragonballs.
Earth's Dragonballs and the creation of the Black Star Dragonballs-
The Earth received it's set of dragonballs when a Namek spacecraft landed in Yunzabit Heights. Inside the spacecraft was a small Namek (name unknown). He grew up in Yunzabit, not knowing who he was or where he came from, until he was old enough to leave. This Namek, through challenges in his life, became the GUARDIAN OF EARTH. During this time, he created 7 dragonballs of his own. However, something was strange about these balls. They had a black star in the center, instead of an orange one.
This particular Namek had an evil being that swelled inside of him. The Namek's feelings and emotions were very turbulant and powerful. Therefore, the resulting Dragonballs were tainted with evil energy. The Namek feared the energy of these dragonballs. Their energy was overwhelming. To ensure no one could ever harness this evil energy, he scattered the Black Star Dragonballs throughout the galaxy. The Namek forced this evil energy out of his body. The energy took the form of another identical version of himself, but of pure evil. Now that the Namek was pure, he made another set of Dragonballs; he succeeded. He decided to model the dragon after the classic Japanese Dragon. These dragonballs were about the size of a large marble; much smaller than the original dragonballs on Namek.
Earth's Second Set-
During Cell's attack on the Earth, the unknown Namek who is now called, KAMI (God), fuses with his other half, Piccolo, that seperated from him so long ago. When he does, the Earth's Dragonballs become useless and turn to stone. However, the Black Star Dragonballs survive because Kami and Piccolo make up the Namek that first created them.
The Earth needed a new guardian. Goku traveled to New Namek and came back with a new guardian, DENDE. Dende resurrected the old 7 dragonballs from stone. He used the previous guardians model of the dragon to go by. Therefore, the new dragon and dragonballs were the same as the previous ones. The only different attribute on the new dragonballs, was the addition of one more possible wish. The Earth now had 2 wishes per summoning at their disposal.