The Dragonballs are Orange glass-like spheres with the ability to call forth a massive dragon, which has the ability to grant wishes. Dragonballs come in sets of seven with each ball containing a star that signifies it's respective number in the set. Dragonballs aren't easy to find because they are scattered all over the planet after every wish, so to have the opportunity to summon a dragon one must travel all over the globe to retrieve them.
It must be said that the dragons are not genies and they can refuse a wish for many reasons, ususally because of power restrictions in the respective dragon. Basically all Dragonballs are the same, however in Dragon Ball GT, possibly the most powerful set Dragonballs can be seen. This special set of Dragonballs feature Black stars (as opposed to Dark Orange) and can call forth a dragon with the ability to cast any wish, with no power restriction. After a wish has been cast with the Black star Dragonballs, the balls are scattered across the universe (rather than planet).