There are a number of Passeriformes (perching birds) called "warblers". They are not particularly closely related, but share some characteristics, such as being fairly small, vocal and insectivorous.
They are mostly brownish or dull greenish in color. They tend to be more easily heard than seen. Identification can be difficult and may be made on the basis of song alone. To English-speaking Europeans, warblers are the archetypal "LBJs" (little brown jobs).