This is a great sonnet for looking at how structure is used to help create and support meaning. This is a classic English sonnet structure -- 3 quatrains and a final, rhyming couplet to provide the conclusion statement. In this poem, the speaker is explaining how true love is constant, and that is the theme of the poem.
To illustrate his point he makes three observations -- one in each of the quatrains:
1. Love is NOT love if it "alters when it alternation finds, / or bends with the remover to remove." This means that true isn't changed by passing distractions or by the absence of the loved one.
2. Love IS a permanant thing that is unmoved by storms (of emotions). It is like the North Star in the sky -- a constant mark used for navigation. It is something worth more than any measure of value that could be assigned.
3. Love is NOT subject to the passage of time and can outlast even death. It will last until the "edge of doom" which suggests the end of the world and the Last Judgement.
So, Shakespeare defines love by what it IS NOT, then what it IS, and finally what it IS and IS NOT together. His final couplet is just a bold statement about how right he thinks his point is -- claiming that if he isn't right, then no man ever loved.
I believe the description is of true love, romantic, familial or platonic. And his first statements admit infatuation is not love. Whether found sensually or intellectually, true loves endurance is boundless.
I also believe he speaks of him self. Knowing he had cheated true love by altering when he found an alteration or deviating from a vow. While being lost in a sea of meaningless relationships his true love was always there to throw him a life ring at the edge of doom. At the end of life or before falling in the gutter, love has always proven to be resilient. Many times battered, betrayed and made the fool, true love is without end.