I've been rummaging around the world of small boats for maybe 18 months now, and have never looked up definitions of these terms. I have tried to distinguish any outstanding features common to each name, and have found none. So today, I started looking online for definitions that would help me understand the differences. Still no closer, and in fact, may be further from my goal.
So, while I think I safely eliminated 'pram' from my initial list due to one single defining hull design feature that can clearly set one apart from any other boat, under dinghy I got "usually from 6 to 20 feet in length". Okay, this isn't helping much!
Under 'skiff' I got the following:
"can refer to high performance sailing dinghy"
"usually a sea-going fishing boat" (in America), and
"a flat-bottomed open boat"
So, one of the first examples I dig out is the Melonseed Skiff. Never a dinghy by design as far as I know, not a fishing boat, and not open nor flat-bottomed (I guess it's really close enough to flat-bottomed, though?).
Now, I'm okay with the definitions that were applied to working boats--and the definition of a dinghy as a sort of tender that's towed, and a tender as any boat whatsoever routinely chosen to tend to another boat, and maybe a skiff as a boat, of any kind, designed and used to haul in fish and be beached rather than moored. I'm choosing especially-loose constraints and saying I'd be able to grasp that such definitions are useful if not precise.
But, do you see a fair consistency in the use of these terms for small goof boats (pleasure craft), and I'm just not seeing the subtle distinction--OR, are the terms applied pretty much as folks see fit to elicit the most romance? I really can't tell with my non-existent experience.