A Howler is a nasty letter sent to tell someone off. It arrives in a red envelope, smoking slightly. It must be opened immediately or it explodes. Upon opening, the Howler screams at the recipient in a voice magically magnified for maximum effect. Students at Hogwarts occasionally receive Howlers from their parents.
After the flying Ford Anglia incident, Molly sent Ron a Howler to tell him off (CS6). (Fear of getting another Howler was what kept Ron from writing home and asking for a new wand to replace the one broken in that incident.)
Neville's gran sent him a Howler after he was punished for losing the passwords into Gryffindor Tower during the 1993 - 1994 [Y13 - Y14] school year (PA14).
Howlers were sent to the Ministry of Magic to complain about the "lax security" at the Quidditch World Cup. Percy had to deal with some of them. He said he had scorch marks all over his desk (GF10).
When Rita Skeeter wrote gossip about Hermione, some readers of Witch Weekly sent Hermione Howlers (GF28).
Dumbledore sent a Howler to Aunt Petunia, reminding her of her obligation to let Harry live at number four, Privet Drive (OP2).
Ron expected to get another Howler after the twins left school (OP30), but didn't.
