The two major ones are called InPrivate Browsing and InPrivate Blocking. The former eliminates browser history, cookies, cached files, and other ordinarily stored information when a browsing session is over.
A variety of implausible usage scenarios are described by Microsoft: looking at banking websites on shared computers or doing Internet shopping to buy gifts without the recipient finding out. The most likely situation, however, is the obvious one. Nudge nudge, wink wink, say no more.Apparently some have dubbed this feature "porn mode." I don't think it's that simple, however. Last year, Google acquired GreenBorder, which created a virtual machine in which a browser could safely run, isolating any malware or security breaches. Users would sign up for an annual subscription.
Some have speculated that it was the first step toward a security product, but generally when Google has acquired products and services, it has left them available to customers. In this case, the company has announced no plans, and if you go to the GreenBorder site, all it says is that it will support existing customers through the end of their current subscriptions. Is there a reason that Google might not want such a service easily available? Sure, it could mess about with tracking user activities over time, reducing the power of targeted online advertising and the rates it could command.
More to that point is InPrivate Browsing: