Not sure what a District Attorney is? Well, if you're an American, you might also know of this post under the title DA, Prosecuting Attorney, State's Attorney (or State Attorney), County Attorney, County Prosecutor, or Commonwealth's Attorney. If you're not American, you probably don't have a DA in your area. Most of the English speaking world will have Directors of Public Prosecutions instead.
Regardless of what title is used, the DA is the head prosecuting officer in a given judicial district. A DA's duties generally include reviewing police arrest reports, deciding if criminal charges should be brought against an arrestee, investigating cases, prosecuting criminal cases in court, and managing the prosecutor's office. This is often too much for one individual, which is why God created Deputy District Attorneys and Assistant District Attorneys.
The job of DA is an elected position. When was the last time you voted in the DA race? It's an important position, no? You don't want just any yahoo controlling who goes to prison for life or plea bargains down to 30 days of community service. Rock the Vote!
DAs come in various levels: a State's Attorney or Commonwealth's Attorney works on the level of the state; a County Attorney works on the level of the county; a City Attorney is limited to the city. County and city attorneys may be limited to trying only non-felony infractions or misdemeanor cases, and are less likely to be referred to as DAs.
And then there are Federal District Attorneys, more properly known as United States Attorneys, which are a different beast altogether. These guys (and gals, of course) act as the chief prosecutor for the United States in criminal law cases, and represent the United States in civil law cases as either the defendant or plaintiff, as is called for. They are appointed by the president, subject to confirmation by the Senate.
So now you know what a DA is. I hope you never have cause to meet one.