Hi, Cody,
I have here a book called "The Tonnage Measurement of Ships". It runs to over 100 pages (plus indices and appendices) and only covers gross and net tonnages, with not a word about deadweight!
How complicated do you want to get: because this can become a really complicated subject?
In my initial post, I tried to sum it all up in just two sentences but Blue Wombat and John Jones have added a good bit of meat to what I said.
John's last two paragraphs contain the meat that I was tempted to add . . . but didn't, in the interests of simplicity.
If you want to know how much coal,ore or grain, etc you can pour into a bulker's holds or how much liquid into a tanker's tanks, you do need to know the ship's cargo deadweight (DWCC). Pour in too little and the owner will lose money; too much and the ship might even sink!
For the ship spotter, gross tonnage is about the best guide you will get to how 'big' a ship is compared to others but, as Blue Wombat says, it is not a lot of help when comparing a cruise liner with a VLCC.
Having worked in the shipping industry, I find it difficult to compare bulker with bulker or tanker with tanker in terms other than deadweight tonnage. Similarly I find it hard to think of a containership in measurements other than TEU.
What I do find, though, is that I get very irritated when some non-specialised, journalist person refers to a ship as WEIGHING x-thousand tons, when they are quoting the ship's gross tonnage!
Hope you are now a much wiser shipspotter. I have always reckoned that the more you know about how the industry works, the more interesting it becomes as a hobby interest.
Cheers
Bob Scott