Simile: A simile describes something or someone by expressing their similarity to something else. A simile always has the word "like" or "as" in its phrasing.
Example 1 From everyday speech:
She looks as white as a sheet. (She may not literally be white, but the expression has implicit meanings which we also understand when we hear this phrase. It is a much stronger form of description than "She looks pale."
Example 2 From literature:
My heart is like a singing bird
Whose nest is in a watered shoot;
My heart is like an apple tree
Whose boughs are bent with thickest fruit;
My heart is like a rainbow shell
That paddles in a halcyon sea;
My heart is gladder than all these
Because my love is come to me.
from A Birthday by Christina Rossetti
NOTE: Christina Rossetti uses three similes in this verse. Look at the things that Rossetti compares her heart to. Does she give the impression of feeling happy or sad ?
The first simile is a singing bird. Consider when people sing - what sort of feelings make you want to sing ? The watered shoot represents plenty - all the things needed in life are there.
What makes the image of an apple tree heavily laden with fruit such a positive one ? We associate different ideas with the image of fruit: youth, health, a land of plenty. The fact that this tree is full of fruit is showing us that he is full of love and emotion.
There is also a sexual connotation to the apple - remember it caused the fall of Adam and Eve as it gave them the knowledge of their own nakedness. The fact that the tree is an apple tree shows us her love as sexual love.
The rainbow, like the apple, is also a Christian image - a symbol of God's love, and in many cultures it is the physical connection between heaven and Earth. The shell is a symbol of femininity, so again we have ideas of God and love and sexuality in this simile.
Christina Rossetti, in her writing, used a lot of symbols to portray her ideas - you don't need to be this complex, but do use similes - make your own connections and use the symbols and images that mean something to you.