The sonnet is a lyrical form of poetry that means that it tells a story, it fosters reflections and it is about personal/private feelings and emotions.
The sonnet originated in Italy especially with contribution of Petrarch in XIV century and it entered England one century later, during the Renaissance.
At the beginning Thomas Wyatt, the first English sonneteer limited to translate Italian sonnets (called also Petrarchan) into English language but then Henry Howard, Earl of Surrey elaborated and adapted them, because English rhythm was different from Italian one.

The first consists in alternation between stressed and unstressed words; the second instead depends on the length of the syllables.
The difference between Petrarchan sonnet and English (Shakespearean or Elizabethan) one is:
• The first is arranged in an octave and a sestet. In the octave a problem or questions are posed, in the sestet the possible solution or answers are given.
• The second consists of three quatrains and a rhymed couplet. The function of the three quatrains covered that of the Italian octave. Therefore the solution is given only in the rhymed couplet in a way that the reader can reflect.
But in order to be a sonnet it should have fourteen lines.
The sonnet is not necessarily about love not returned.

SHALL I COMPARE

Shall I compare thee to a summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date:

Sometime too hot the eye of heaven shines,
And often is his gold complexion dimmed;
And every fair from fair sometime declines,
By chance, or nature's changing course untrimmed;

But thy eternal summer shall not fade,
Nor lose possession of that fair thou ow'st;
Nor shall death brag thou wand'rest in his shade,
When in eternal lines to time thou grow'st:

So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee.

"Shall I compare" is a Shakespearean sonnet because as you can see it is arranged into fourteen lines: three quatrains and a sestet.
It belongs to a collection because the title resumes the beginning of the work, indeed it is William Shakespeare's eighteenth sonnet.
In the three quatrains the author tries to make effective the comparison between a young and the summer (also with the sun: when a poet compares something to the sun it means that he compares something beautiful).
Nobody knows who is the young (the fair youth) whom Shakespeare refers; therefore it is indicated only with the initials of his name: W.H.
Mr. W.H. turns out to be better in any sense and hence we will not expect the comparison in this way because Shakespeare points out summer's imperfections.
In the first part of the sonnet imperfections about the temperature are fore grounded, in the second one those about the length.
In the third quatrain, instead, the focus on the summer changes and the young becomes the subject. In this part Shakespeare wants to underline that "the fair youth" may become eternal thanks to his verses. Therefore the problem posed in the sonnet is to avoid the death and the immortality.