The biggest difference? Cost.
And believe it or not, small tanks are not the way to start. They are death traps for fish. I would not have anything smaller than 40-50 gallons.
tank and basic filtration is the same, although I would suggest using external canister filters, a protein skimmer is highly recommended if not required, as is a wet/dry system with bioballs. It is also very important that you get live rock and/or live sand. It helps in the biological filtration and in keeping water quality good.
Your water chemistry has to be dead on. The fish are not as tolerant of quality changes as freshwater fish, although they can be remarkably resilient. I would also suggest starting with a fish-only system and avoid the temptation to get into reefs/invertebrates until you are much more comfortable with marine tanks and can afford the lighting and extra filtration.
There are some good books out there for beginning marine types. I know there is a book thread going on at the board now, and I recommended The Conscientious Marine Aquarist by Robert Fenner. You might want to check it out and read up a bit before making the plunge.
All things being equal, if you're fairly comfortable with your freshwater setups and maintaining them, the transition can be nice and easy as long as you do some planning ahead of time.
Good luck! You'll love it!
Devin
And what would I need to start it all off?
