In literary criticism, a symbol is an image with an indefinite range of reference beyond itself Some symbols are conventional ("the sun," "the eagle," "the Good Shepherd"), as they have a range of significance that is commonly understood in a particular culture. Other symbols are private or personal, having a special significance derived from their particular use by an author.
The simile draws a limited comparison between one thing and another ("as fierce as a tiger"--fierceness is the sole quality compared). The metaphor is more open; it states a likeness by implication, leaving it to the reader to make the connection.
When the young Shakespeare gives the Duke of York, who is being taunted by Queen Margaret, the line "O tiger's heart wrapped in a woman's hide!" his meaning depends on the particular qualities associated at the time with the tiger: that they included not only fierceness but also deviousness is shown by the infamous parody of the line by his jealous contemporary, Robert Greene, who warned others in the literary world that Shakespere, had a "tiger's heart wrapped in a player's hide."
The symbol is yet more open. Blake's tiger is both fierce and beautiful:
Tiger Tiger, burning bright
In the forests of the night,
What immortal hand or eye
Dare frame thy fearful symmetry?
And as the poem develops the symbol becomes increasingly various and complex in its reference, an exploration of the nature of the tiger's creator:
When the stars threw down their spears
And watered heaven with their tears,
Did he smile his work to see?
Did he who made the Lamb make thee?
Writers can develop symbols created by others, and can reinterpret conventional ones; Adrienne Rich, in "Aunt Jennifer's Tigers" counterpoints the energy and beauty of a Blakean tiger with the symbol of a wedding ring in such a way that the conventionally threatening tiger becomes attractive as it "paces" in "sleek chivalric certainty," and the conventionally desirable wedding ring becomes oppressive as its "massive weight . . . / sits heavily" on Aunt Jennifer's finger.