This is because grand jury proceedings are rather one-sided: there is generally no judge involved, nor any defense, but rather the prosecutor simply presents whatever evidence he or she chooses, and has to convince the grand jury that there is "probable cause" that the person committed a crime, i.e. that a reasonable (ordinary) jury could conceivably convict. If this seems like a rather low bar to you, you're right: quite a few people have argued that grand juries are a complete waste of time, and only half of US states still use them. (The federal government is required to by the fifth amendment; no common-law jurisdiction outside the US still bothers)
In those places which still use them, their main remaining function is to provide plausible deniability to prosecutors who don't wish to pursue a case: just like you could get a grand jury to indict a ham sandwich by only showing one story, you could get a grand jury to refuse to indict Freddy Krueger by showing them enough evidence to make the legitimacy of the state's case unclear.
That's not a common use for grand juries -- prosecutors generally have better things to do with their time than look for plausible deniability. (In the federal courts in 2010, for example, grand juries refused to indict 11 times, out of about 162,000 cases. Given that a prosecutor can generally guess when they don't even have a good enough case to indict, you can assume that those eleven each decided to have the grand jury be the one to say no, instead of them, for a reason)
What this means is that when you're trying to interpret the news and understand what a grand jury verdict means, you can basically take it to be a summary of the prosecutor's decision to prosecute or not to prosecute the case, rather than the verdict of an independent panel.
(The analysis below notes that, in high-profile cases, there's another important reason that a grand jury may not indict, which is that the prosecutor feels that the case isn't strong enough to actually push through, but nonetheless feels political pressure to try anyway. That's not likely to be the case with today's news, as county prosecutor Bob McCulloch took the rather unusual step of having Darren Wilson, the prospective defendant, testify before the grand jury for several hours. Prosecutors who actually want an indictment generally don't invite the defendant to give their side of the story at length, as this is not considered conducive to getting the desired variety of ham sandwich. So it's fairly safe to read today's headline as "McCulloch decides not to prosecute Wilson," and interpret that as you will.)
If you want to read about the grand jury system in the US, as good a place to start as any is