They sacrificed themselves to further mankind.
From the NASA tweet:
"Today NASA pauses for a Day of Remembrance to honor the fallen heroes of Apollo 1, Challenger and Columbia"
You can read more about the 'Day of Remembrance' here (this year) and here (DoR in general).
Also, in case you didn't know, yesterday (January 28th) was the day the Challenger shuttle exploded in 1986.
Man, I remember that very well. I was 7 years old at the time but it just left such a huge impression on me, I won't ever forget it.
The Challenger crew (picture shamelessly stolen from Universe Today)
I remember seeing the news and they just kept showing the video over and over and over. I can't imagine what the coverage was like in the US, but over here it was crazy; everyone talked about it.
You can read the Phil Plaits take on it here or read more about in on Universe Today.