I know pretty much nothing about DB, but have listened to a younger cousin go on about it enough to be able to tell you that Dragonball came first, and is about the main character as a kid trying to collect dragonballs at great peril, I also know enough to say that dragonballs, contrary to popular belief among the non initiate, are not actually dragon's testicles. :)

DragonballZ features the main character all grown up and with a wife and family of his own.

So, Dragonball came first, followed by DragonballZ, followed by DragonballGT.

Sounds like a line of automobiles, doesn't it?