Simile is a figure of speech used as a comparison and similarity tool. In comparison to metaphors, similes are made with the help of "as" or "like". The simile examples from the book are: 
Say thou wilt course, thy greyhounds are as swift 
 As breathed stags, ay, fleeter than the roe.
The lines describes the greyhounds like breathed stags and fleeter than the roe. 
And till the tears that she hath shed for thee 
 Like envious floods o'errun her lovely face...
In these lines, the tears are compared to floods.