Could you explain dual mass flywheels to me?

[Ari]

There was an interesting post a little while ago about swapping a dual mass flywheel for a single mass, and replies suggesting this was a bad idea.

I'd heard the term before but have never understood what it is or why the need. All I know is that they're typically fitted to diesels and they're expensive.

So what do they do, and how (if at all) should one modify their driving style to prolong their life?

[Ari]

Certainly no intention of changing it for a solid one, just keen to know how best to treat it to get long life from it, bearing in mind the stories of expensive repair.