It is about the only wood that if left untreated in an exterior marine environment will weather out to a nice look. Most if not all other woods will deteriorate quickly.
Be aware that Teak is very soft and easily damaged by shoes, scrapers etc.
It is expensive, it is pretty and it is brittle as it dries out. It gives a boat owner something to finish with oil, varnish, Cetol, Bristol Finish etc., or nothing. It is traditionally used on boats as it is well suited for the marine environment.
Did I mention that it is expensive? Marine teak costs even more.
It's a conspiracy by boat products companies, WM and other chandleries to keep us perpetually buying and endless array of teak cleaners, varnishes, oils, urethanes, etc. etc. and forever chat and blog about our choices with fellow boaters!! :laugher
some information about Teak from Wikipedia. it is classified as a hardwood because it is a deciduous tree. whether or not the wood is actually "hard" or "soft" does not matter in the classification. it just tends to be that deciduous trees are harder than coniferous trees.
Teak is about the same hardness as Honduran mahogany, which is to say medium density. Good strength-to-weight. It sands easily but can be difficult to cut (and hard on tools) because teak grows in silicaceous soils and takes vitreous silica up into the wood. Throws off sparks, too. But the wood itself is of modest hardness compared to, say, jatoba or locust. There is a substantial hardness difference between annular earlywood and latewood, which causes a 'corduroy' effect similar to gymnosperms like douglas fir. So it's famously non-skid.
Teak is prone to grain checking and suffers rollover failure along crisp edges, but it generally doesn't exhibit running splinters like oak, cypress, or phillipine mahogany (shorea spp.).
Along with oils, which are common in tropical species, teak contains waxy nodules in its grain called tyloses; these help with water resistance and are found in many rot-resistant woods like mahog, white oak, and black locust. More info that you really wanted, eh?