Loose sentences:

The main point comes early in the sentence.  Phrases and subordinate clauses that modify the point follow.  

Loose sentences seem to resemble the way we speak, with thoughts coming as they occur to us.  They are far more common.

MR. UTTERSO, the lawyer, was a man of a rugged countenance that was never lighted by a smile; cold, scanty and embarrassed in discourse; backward in sentiment; lean, long, dusty, dreary, and yet somehow lovable.

-Robert Louis Stevenson, The Strange Case of Dr. Jekyll and Mr. Hyde.  

She was a woman of mean understanding, little information, and uncertain temper.

-Jane Austen, Pride and Prejudice

The man who was sent to the office would linger about the place long enough to get the drift of the conversation from the group of white people who naturally congregated there, after receiving their mail, to discuss the latest news. 

-Booker T. Washington, Up from Slavery

Periodic sentences:

In a periodic sentence, the punch line is saved for the end.  A periodic sentence begins with modifying phrases and/or clauses and ends with the main thought, the independent clause.  It is less common, harder to write, and harder to understand, but, if not overused, emphatic.

When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.

     -Thomas Jefferson," The Declaration of Independence"

WHEN I had finish’d my dinner, and drank the King of France’s health, to satisfy my mind that I bore him no spleen, but, on the contrary, high honor for the humanity of his temper—I rose up an inch taller for the accommodation.

-Lawrence Stern. A Sentimental Journey through France and Italy.

If, instead of listening to the war-mongers of the military-industrial establishment, the politicians had only listened to what people had been writing in their letters and in the newspaper columns, if they had only listened to what the demonstrators had been shouting in the streets and on the campuses, if they had only listened to what was in their hearts, the war would have ended long ago.

- John F. Kennedy

In order to defend and protect the women and children who were left on the plantations when the white males went to war, the slaves would have laid down their lives.

-Booker T. Washington, Up from Slavery

Son of a potter, through all the stages of his fortunes he led a foul life.

-Machiavelli, The Prince

To these partial anticipations, however, Darwin owed little.

-Introduction to Origin of the Species