Not that I'm at expert but here's my take on it.
Dry Camping......... you are using all the self contained items on you RV. ie water pump, gray & black tanks, propane, batteries, might also have solar and or generator.
The spot you are in is laid out, may have table, BBQ, or fire ring. Water may or maynot be around but would need to hand carry back to you spot. Has outside bathrooms, but could be pit toilets. Might also have a dump station. There is a small cost to stay there. Most likely the sight is a National Park, National Forrest, BLM campground. Could also be at a County Campground.
Boondocking........... like dry camping you are using all the self contained items.
Difference is spot isn't laid out. Could be in a parking lot, rest stop, out in the desert, in the mountains, down by the beach. No services unless in a WM, or truck stop parking lot. Most are way off the beaten path. No cost to stay there.
What have I done more of............... Dry Camping but every year for the past 20 or so we go out to Quartzsite & boondock. Did that for Thanksgiving last year. Thinking we will do it again this year.
Also do it in the middle part of January when the desert is packed with RV's from all over the US & Canada.
Not sure how true but we have heard that Quartzsite becomes the fourth largest city in Arizona at that time.