Similes, Metaphors and Allegories are essential tools in writing lyrics where the intent is to leave the lyric more open to interpretaion by the listener.

So, I thought I'd bring together the descriptions in one place. I've reworded the best I found on the web in the hope that the meanings are clear.

Simile

a simile states that A is like B

A simile is a juxtaposed comparison of two or more subjects to draw attention to their similarities. Similes are typically identified by the use of "like" or "as" or "than", or "resembles". Similes are a specific and formulaic form of allegory.

Metaphor

a metaphor states that A is B or substitutes B for A

A metaphor is defined as an indirect comparison between two or more apprently unrelated subjects that typically uses "is a" to join the subjects

Allegory

Allegory is a type of extended metaphor, where objects, individuals, and actions in a work, are equated with meanings outside the work itself. The hidden meaning has social, moral, religious, or political significance, and often characters are personifications of abstract ideas such as greed, or envy. Allegory encompasses such forms as fable, parable, and apologue and may involve either a literary or an interpretive process.

An allegory is a story with two meanings, a literal meaning and a symbolic meaning.