A Howler is a type of owl post. It can 'record' a person's voice and when it has been delivered, the recipient has a limited amount of time to open it voluntarily before it will burst into flame and scream its contents. It is called a Howler because it quite literally howls its message.
From the Story
"A Howler is a magical letter that comes in a red envelope and, if not opened immediately, smokes as though it is about to explode. Once opened, the voice of the sender will erupt from it, shouting at the recipient very loudly."
People who have been known to receive Howlers
- Neville Longbottom mentioned receiving at least one howler from his grandmother.
- In Chamber Of Secrets, Ron Weasley received a Howler from his mother after he stole his father's Flying Ford Anglia.
- Neville Longbottom received a Howler from his grandmother, after he supposedly misplaced his password list enabling Sirius Black to enter Gryffindor Tower.