The Art of Shakespeare’s Sonnets

What is meant by 'sequence' in respect of the Sonnets?

Many critics believe that the sonnets were written in a certain order...a sequence...which was later lost in the shuffle.  Some folks have spent much time and energy trying to figure out when and in what order the poems were written in order to shed a little more light on Shakespeare's life and what his personal thoughts/opinions on particular subjects were without having to glean this from characters in his plays.  It is generally believed that the speaker in the sonnets is the poet himself and not some contrived character.  One person of particular interest in the sonnets is The Dark Lady--who she was, what importance in his life she had, and whatnot.  The numbers given to the sonnets are not necessarily the order in which they were written, but more or less to keep up with how many poems/sonnets were attributed to the author. 

In reply to #2:

Thank you, Amy-lepore.