Sonnet 18 is of Shakespeare's most famous sonnets, and one whose themes and many quotes from it have been absorbed into the language, not least its famous opening line, "Shall I compare thee to a summer's day?".
It has many themes in common with Shakespeare's other sonnets, and I've put the themes in bold for you. It begins with comparing the beauty of a woman to the beauty of nature in detail, and despairing at the transitoriness and changableness of summer and nature (the cyclical, changing nature of nature). Even the sun, the speaker laments is changing - and he concludes that everything in the world changes:
And every fair from fair sometime declines,
By chance or nature's changing course untrimm'd
However, the beauty of the woman will not fade - and, moreover, beauty will survive even death because the poet has preserved that beauty in writing.
The final couplet concludes that, as long as men can breathe or eyes can see, his writing can be read - and therefore the woman's beauty is eternal.
Hope this helps!
love all the things and person you have...