North Korea's an interesting one. In short, they are a deeply screwed up country on any metric you care to choose (but mainly economically) trying to carve out a niche in a post Cold War world - they used to rely heavily on Soviet aid, just as the South Koreans rely heavily on US support but unfortunately that's all gone.
Their leader, Kim Il Jong, has to be considered somewhat doolally but arguably isn't quite as mad as he comes across but rather is trying to negotiate a future for his country in a cack-handed, authoritarian, stupid and reactionary manner. About the only thing he can do to get any reaction at the negotiating table (hell, to even get to the negotiating table is threaten nuclear capability, or to give up potential nuclear capability - he hasn't got anything else, no money, no exports, no influence, no allies. I guess it also pushes up his poll rating at home, not that polls are that important over there, but furthers the cult of the personality anyway.
Every now and then he seems to push this button harder than usual, usually to get the attention of the US and China. What usually seems to happen (and will probably happen this time) is general condemnation, followed by language around, and possibly even action around, increasing the level of economic sanctions against North Korea. Probably moves them up Obama's to do list as well. Most importantly, will encourage more active engagement with the North Koreans in which they'll use this threat to try and get some benefits for themselves at the negotiating table - I presume lifting of sanction, trade agreements etc, anything that stops their country going bankrupt and their people starving.