DefenseLINK News: Iraq Attacks Lowest Since March 2004, Officials Say

Iraq Attacks Lowest Since March 2004, Officials Say: There are between 40 and 60 incidents each day in the country, they said, sharply down from the terrorist effort in the week of the Iraqi elections in January.

Even this doesn’t tell the whole story. Of those incidents, roughly half have no effect. This means terrorists launch an attack, but no lives are lost, nor is any property damaged.
But “let’s not downplay this,” said a senior defense official. “Some of the attacks are horrendous, like the attack on the mosque in Hillah that killed more than 100 Iraqis.” (emphasis mine)
I'm confused by an attack that does no property damage and kills no one. What exactly happens during one of these?