The Ghostbusters International Edition actually has an intro which does that:
So, does this Ghostbusters roleplaying game differ substantially from
the one that hit the shelves all those years ago? We're glad you asked
that question. This one's got a few more rules, especially about who
can do how much of what to whom. You asked for 'em. We wrote 'em.
And this one tells you about nifty, previously unquantified things
like how much your equipment might slow you down when you're chasing
(or being chased ) through Central Park in the hot summer sun with
three loaded ghost traps, and a proton pack, and a really big
flashlight, and a weather radio, and ... and all that stuff.
You asked for details, so we've got droves of details: there 's a Big
List of Equipment, and a section about accidents and damage incurred
there from, some new stuff about campaigns and adventure design and
putting together really fast spooks. We figured you'd need to know how
to distinguish between an intelligent specter and a mindless one, a
physically manifested ghost and an ethereal icky-so we mad e up a
nifty little table to do the job.
Ghostbusters International gives you the inside poop on the franchises
that our fearless New York office set up at the end of the first movie
... their heyday, their languishment and their triumphant revival.
In my words though the two editions are similar to the changes between OD&D and AD&D. The bare bones of the system are largely the same but the major changes have to deal with the game's new subsystems like gear, encumbrance, magic, and weird science. The second edition also does a much better job of clearly explaining both the rules and how the game is played. The first edition is a little more gimmicky, your character sheet is actually your Ghostbusters ID and you "making your character" is you filling out the details on your Official ID Card which starts with you adding your picture.
Ghostbusters international, the later edition drops that gimmick (mostly) and is very clearly an RPG with a more standard layout, format, and character sheet. As mentioned before the rules are more clear in this edition with very clear sections breaking down creating a character, playing the game, gamemastering, and running an adventure. A bunch of the International edition's focus on what happens when the characters aren't out busting ghosts. It introduces a whole section for what it calls Routines, essentially a bunch of random charts for dealing with stuff like Car Chases, getting Permits/Legal documents, going to Court, Research, etc. The gamemaster section also contains a ton of details on how to run a campaign with advice on how to capture same feel of a ghostbusters game with sections on Contacts, Running Gags, and Major Villains.
Back out from behind the screen a great way to see the mechanical differences is comparing the character sheets.
As you can see the core mechanic of the system, Traits are the same. They're both how you resolve skill checks and how you take damage. When rolling a skill you roll a number of dice equal to the relevant trait with 3 bonus dice if you have a talent for the associated trait related to the current check. Everytime you roll a check though you also roll what's called the ghost die which can add complications to the circumstances of a roll. If the test succeeds and you end up rolling the ghost then there's a twist to your success. If you fail you fail badly.
What's also the same are brownie points which are a pool of points players can use to boost rolls, avoid bad stuff, and twist the story a bit. They're also used like experience and can be spent to permanently increase traits, buy talents, and gain new abilities.
What's new is Medical History which determines how severely injured you are after an encounter to determine the level of care required to heal them and equipment (as mentioned earlier) which provide bonus dice to rolls or grant special abilities or effects.
