You asked two questions and so according to enotes regulations I have been forced to edit your question down to focus on just one question. This is arguably one of Shakespeare's most famous sonnets, immortalised by its use in the film production of Sense and Sensibility starring Emma Thompson and Kate Winslet. This sonnet defines love metaphorically as "the marriage of true minds" and argues that the key characteristic of love is its constancy in the face of fading beauty or the passage of time. The sonnet ends by the speaker saying that if he is wrong in what he writes then he is no writer and no one has ever loved.
Interestingly, the poem has examples of defining love by talking about what it is not and what it does not do. For example, it says:
Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove.
Love's not Time's fool, though rosy lips and cheeks
Within his bending sickle's compass come.
All of these negative definitions of love serve to emphasise the constancy of love and how it remains unchanged in spite of ageing and death. Thus the sonnet doesn't really describe a relationship as such--it describes the true nature of love.