Hi South,
Fundamentally its the same principal but it significantly more involved. Yes you have to add salt, but the water has to be distilled or reverse osmosis water. You also have a wider range of parameters to measure, ranging from ammonia, nitrite, nitrate, calcium, phosphate, ph, alkalinity, to name just some. Unlike freshwater fish that you can just mix with a de-chlorinator, marine fish require pure water mixed with sea salt. Tap water has around 700 chemicals in it, including a lot of disolved salts and often a high nitrate content.
Firstly you'd need to decide whether or not you want a FOWLR (Fish Only With Live Rock) or If you intend to have a reef setup. FOWLR allows you in theory to choose from a wider selection of fish. Some fish will feed upon corals and so wouldn't be compatible with a reef setup. Some fish also prey on invertibrates and so wouldn't be able to be kept if you had shrimp in your tank, or vice versus.
In a marine tank you have live rock. This is just pourous rock that contains nitrifying bacteria and other marine life and forms the basis of the filtration system in your tank. The rock is placed in the aquarium and left to cure. During this time, some organic life that was on the rock will die back, due to transportation mostly. This creates ammonia and starts off the nitrogen cycle in the tank and so has to be cycled like a freshwater aquarium does.
However in a freshwater aquarium, mechanic filters are used, either internal or external, to house the bacteria. In a marine setup, an external filter isn't acutally required, the rock providing a home for the bacteria, and kept alive by flowing water provided by powerheads/submerged pumps. Once the tank is cycled, and ammonia, nitrite and nitrate are all at 0, you then add your CUC or clean up crew.
CUC's consist of critters such as a variety of snails, shrimps, hermit crabs and starfish. These will be essential to the ecosystem by eating algae, detritus and scraps of food missed by fish. You can also add macro algaes to reduce nitrates and phosphates, although these will require good lighting with T5 bulbs recommended. Once the clean up crew has done its job, you can then start to add fish.
Equipment you will need consist of a heater, powerheads, protein skimmer and light units. You will also need a refractometer for measuring the salt content of the water, as well as a variety of water testing kits to monitor the water chemistry. Once the tank is well established, you can just occasionally monitor the water for specific elements like calcuim (required for corals). You could also have an external filter if you so wished, as they do provide a chamber in which you can add reactive elements like active charcoal or phosphate/nitrate absorbants etc. I have one connected to a vecton UV sterilizer. Again this is optional and not essential.
I'd suggest you read up on such things as live rock, live sand and the task they perform. What a protein skimmer is and what it does. Also the water chemistry and desireable parameters to achieve; like a PH of 8.3 and SG (salt gravity) of 1.021-1.026 for example. If keeping invertibrates the SG should be around the 1.026 mark. Planning is the key thing and researching everything. You are essentially recreating a little eco-system and to achieve an equilibrium within it. Therefore things need to be compatible, from fish to invertebrates to plants and corals.
I am a beginner myself, but keeping tropical fish has helped greatly with understanding the basic concepts. If I'd just gone straight for marine fish without having had any prior knowledge of what was involved, I'd be in allsorts of trouble.
Hope that helps,
AK
) i have spent about $2000 on my 55 (us) gallon reef tank...only have live rock in there atm...no fish...no inverts...no cuc....it can get very expensive very quickly...and mistakes are COSTLY...lol...that's why it's best to do your research. 
