All Saiyans have a monkey-like tail that allows them to transform into a Great Ape when they look at a full moon. Saiyans who lived on Planet Vegeta, like Bardock, Paragus, King Vegeta, Vegeta, Nappa, and Raditz, wear their tail wrapped around their waist in a belt like fashion. This custom is likely to keep their tails from being reached and gripped easily by opponents. When a Saiyan's tail is squeezed, it causes them agonizing pain, though Nappa and Vegeta become immune to this. Goku also became immune to this weakness, shown when Krillin attempted to squeeze his tail, to no avail. If a Saiyan's tail is cut off while they are in Great Ape form, they revert to normal.
Most of the Saiyans in the anime get their tails removed, but on some occasions, they grow back. The Daizenshuu description is that the tail will often suddenly grow back if the Saiyan is in danger; such as when it did during Goku's match against Giran or for Gohan during the battle against Vegeta.
Goku uses the Super Saiyan transformation while at the same time possessing a tail in Dragon Ball GT, but sometimes his tail does not change color to gold. The fur is still brown when Bardock turns Super Saiyan in Dragon Ball: Episode of Bardock. However, when Goku turns Super Saiyan 3, his tail fur always turns gold. When Goku and Vegeta turn Super Saiyan 4. their tail furs both take the color of their respective Super Saiyan 4 fur.