Porunga is the Namekian dragon that emerges from the Namekian Dragon Balls. He can grant three wishes instead of one, and you must be speaking Namekian in order for the wish to be granted.
Porunga is the Eternal Dragon of Namek, summoned when all of the Namekian Dragon Balls are gathered. Also, since a year on Namek is only 130 days (as opposed to a year more than twice as long as that on Earth), the Namekian Dragon Balls can be gathered more often than the ones on Earth, and thus, Porunga can be called on more often than the Eternal Dragon of Earth Shenron can.
As far as physical characteristics go, Porunga is much more vicious-looking than Shenron, with a heavily muscled upper body (large pectoral muscles, shoulders, arms, and a well defined four pack). Instead of looking snake-like in appearance, like Shenron does, Porunga appears to be more alligator-like, bearing great resemblance to the Naga, a group of serpent deities in Hindu and Buddhist mythology. He has a long green slithery tail instead of legs. He also has a large fin down the center of his back, and large spikes on his shoulders and head, as well as two antennae on his forehead similar to the Namekian people.
Like Shenron, Porunga is good-hearted but being that he is a dragon, he has a temper and sometimes comes off as impatient. He is shown to have a sense of humor, as when he was kept waiting by Krillin, Gohan, and Dende, who had summoned him on Namek, he told them if they couldn't think of what to wish for, "just wish for nothing three times so I can go!". However, he is more friendly and patient than Shenron, and has demonstrated a greater eagerness to grant wishes; he restores Krillin's clothes as good as new as a special service, and is even shown to be happy for everyone after Tien Shinhan and Chiaotzu are brought back to life. In the anime, he saves Yamcha, Tien Shinhan and Chiaotzu the trouble of having to traverse the Snake Way again and returns them directly to Earth when they are wished back to life. He also shows embarrassment, when Bulma tells him he is "kinda cute".