To be classed as a ballad, a poem or song has to be a narrative -- that is to say, it must tell a story, rather than focusing on abstracts or emotions. There is no required subject matter, but they're often concerned with love and romance, politics or are written to be humorous. They tend to be written in the third person, except when speaking from the perspective of a character from the ballad.
The writers of ballads often used simple language, as they were writing for listeners and readers who weren't specialists and wanted their poems or songs to be easily understood. More recently, people sometimes write them as a reaction to other poets making their work deliberately hard to follow or understand. They also tend to use repetition, in the form of a refrain or incremental repetition, to emphasize certain parts of the story.
A ballad is typically structured in four-line stanzas. The second and fourth lines of each stanza should rhyme; rhyming of the first and third lines is optional, so they follow either the rhyme pattern "abcb" or "abab." There is no set length, so the number of stanzas can vary greatly. Dialogue is sometimes incorporated into a ballad, in order to tell parts of the story from a particular character's point of view.
The murder ballad is a form of ballad that dates back to the 18th century. They follow the conventions of a regular ballad, except they're written on the topic of murder. Original murder ballads from the 18th and 19th century were often written as a supplement to a newspaper report on a murder. More recently, many musicians have written murder ballads as songs. Most notably, Nick Cave and the Bad Seeds released an album entitled "Murder Ballads," containing 10 songs written in this form.