NSL's are investigative tools used by the FBI to "fight terrorism". They go to an internet service provider, or a bank, or a library, and slap down the letter requesting information about a customer/client. The letter not only requires information, but is also a gag order on discussing it.
This particular case has a heck of a twist. An ISP was issued an NSL and appealed. "After the telecom challenged the NSL, the Justice Department took its own extraordinary measure and sued the company, arguing in court documents that the company was violating the law by challenging its authority."
The EFF was working with the ISP to challenge the NSL ASAP, IYKWIM. And a judge said there were lots of problems with them and has banned them, but the government has 90 days to appeal to the 9th Circuit.
I'm sure this will end up in the SCOTUS, one way or another. And it really needs to be exposed there, because unlike a search warrant, the FBI doesn't need a judge or demonstrated probable cause to get an NSL. They've been abused in the past, they're a bad tool, and they should be banned.
http://www.wired.com/threatlevel/2013/03/nsl-found-unconstitutional/