The Koran is a collection of revelations given to Mohammad by the angel Gabriel.
The Hadith is a collection of anecdotes about Mohammad, and sayings of Mohammad.
The Sira is the biography of Mohammad.
The Koran cannot be understood without reference to the Sira and the Hadith because many of the revelations in the Koran refer to incidents that were happening at the time of the revelation in Mohammad's life.
If you want to become acquainted with Islamic doctrine, become acquainted with these three Islamic sources.
The best versions of these books for Westerners can be found at CSPI (The Center for the Study of Political Islam). See their bookstore here.
For further reading, see: What Makes Islam So Successful?