A Russian "Proton-M" rocket carrying three satellites exploded today less than a minute after launch from Kazakhstan. Although this has been a fairly rare event for this booster type, in years, there was another in August of 2012.
Russian rocket crashes shortly after launch; troubled space program under fire
However, the Proton was hardly a 'success story' in the first few years of its existence; many launches of the device had problems such as engines shutting down in mid-flight and fuel leaks. The state of affairs was so bad that many people in the Soviet space business were calling for the program to be suspended. Eventually things lined up properly in a row and a trusted payload delivery system was in place.
In the Toronto Star article, linked above, there is mention of a toxic cloud caused by the spilled burning fuel from today's failed launch. The Proton uses hypergolic fuels (in the UR-500's case, unsymmetrical dimethyl hydrazine and nitrogen tetroxide) or "storable propellants". What this means is the fuel can sit in an idle rocket over some time and, unlike cryogenic-based engines (using liquid hydrogen and liquid oxygen, for instance), an 'ignitor' is not needed -- the two fuels just have to touch each other to start the burning process. However, these chemicals are highly corrosive, and extremely toxic to carbon-based-units (humans; us!), which is why a vehicle like the Proton is not 'man rated'. (There have been exceptions: The American "Titan II" booster utilized storable propellants and it was used to launch astronauts as part of the Gemini Program.)
To launch humans into orbit, the highly reliable "Soyuz" rocket is used. That's a great machine. (Watch one blast-off!)
By the way, rockets do blow up, the odd time -- it's to be expected. A functioning rocket engine contains a controlled explosion, after all. But when the rate-of-failure starts to climb, then there is cause for some concern.
Here is one of many videos uploaded to Youtube on today's Proton-M failure...