Zdoom vs Boom ports?

What is the main difference?

I notice most wads with prboom, so I'm not sure why someone uses zdoom?

It's a silly question, but I've never been certain

You can play PrBoom mods with ZDoom (exceptions are exceedingly rare). You cannot play ZDoom mods with PrBoom.

The two ports have very different design goals. You should read up ZDoom, PrBoom, and PrBoom+ on the Doom wiki; and maybe take a look at the ZDoom wiki.

Got it... thanks!

I discovered PrBoom+ today... I've been using PrBoom, but like to use the latest and greatest.