In humans, we have two sexes, and they're comparatively simple: XX, and XY. Everybody gets an X chromosome from their mother, and either an X or Y from their father. The Y is dominant, so that if you get a Y you're male. Turns out this isn't what all animals are like.
Birds are WW and WZ: males are WW and everyone gets a W from them. Females are WZ and chicks get either a W or Z from them, with the Z beind dominant and turning the chick into a female. 
Platypus are even more complex, bearing not only the X's and Y's of mammals, but an abundance of them, AND the Z from birds. From what I can tell, there's different flavors of the X's and Y's too, something like XxXxXYyYyZ, with a total of 10 sex chromosomes, rather than our two. 
Some fish have yet a third system with X, Y, and Z: a Z makes you female; if no Z, a Y makes you male; if neither (XX) you're female. An XX & YY mating produces only males; ZZ's can't happen; XX & XZ yields only females. 
But of course, no matter the wonders of the universe, and the complexities of biology, someone says it's yet another sign of ID. Sad, really. Apparently IDers don't understand the concept of convergent evolution: "If sex evolved, then it had to evolve many different times in many different ways."
 Exactly! They also fall into the typical trap (and ID fallacy) of assuming that "no explanation yet" means "no explanation ever" - they quote another author as saying "Biologists have been spinning theories about just what that advantage might be for quite some time, but so far there is no single clear answer," and claim this is biologists admitting that evolution is impossible.  *Tsk* These people need to learn a bit of logic.