Bone fractures happen when a bone is damaged by stress. The bone can be fractured in many different ways. 

Most human bones are strong and do not break when hit by strong impacts or forces. However, if that force is too powerful, or there is something wrong with the bone, it can fracture.

Types of fractures 
There are many different types of bone fractures.  Fractures are often separated into categories.

Open or closed 
In an open fracture (also called a compound fracture), the broken bone breaks through the skin.  This can cause infection, since the skin usually protects the body from germs.  If the skin is broken, germs can get into the body.  Open fractures can also damage muscles, tendons and ligaments if the sharp ends of the broken bone go through them.

In a closed fracture, the broken bone does not break through the skin.

Simple or complex 
In a simple fracture, the broken bone is the only thing that is damaged.

In a complex fracture, the sharp ends of the broken bone damage the soft tissue around the bone.

Composite fracture 
This is the type of injury in which the broken bone has passed through the skin, forming some wounds. That is why it is also called an open fracture or composite fracture. Compared to the simple fracture, the open wound has made it even more dangerous since the person suffering from it is exposed to infection. Therefore, an operation may be necessary to prevent the wound from becoming infected.

Complete or incomplete 
In a complete fracture, the bone is broken all the way through.  The broken bone is in two separate pieces.

In an incomplete fracture, the bone is not broken all the way through.  The bone is partly broken, but it is still in one piece.  There are a few kinds of incomplete fractures, including:
 Hairline fracture: Only the outer layer of the bone is broken.  This is also called a fissure fracture.
 Greenstick fracture: These fractures usually happen only in children, because their bones are more flexible than adults'.  In a greenstick fracture, only one side of the bone is broken.  The bone is usually "bent," and is broken only on the outside of the bend.

Other types 
In a comminuted fracture, at least three pieces of a bone have broken off.  This means the bone is in four separate pieces or more (the main bone, plus the three or more pieces that have broken off).

Sometimes people have multiple fractures.  This can mean two different things:
 One bone is broken in two different places, or more; or
 One person has fractures in many different bones, all caused by the same injury (like being hit by a car).

On average, it usually takes 6-8 weeks for a fracture to heal.  However, healing times depend on many different things, like which bones were broken, how bad the breaks were, age (younger people usually heal faster), and nutrition.

Diagnosis
Most fractures are diagnosed by using an X-ray.  If the X-ray cannot see the fracture, other scans, like MRIs and CT scans, can show that a fracture exists.

Treatment

The best way to "fix" a bone fracture is to make sure the ends of the broken bone are lined up and tight together.  As the American Academy on Orthopedic Surgeons says:
"All forms of treatment of broken bones follow one basic rule: the broken pieces must be put back into position and prevented from moving out of place until they are healed.... Broken bone ends heal by "knitting" back together with new bone being formed around the edge of the broken parts."

To help decrease pain from the fracture, doctors can prescribe medications or suggest over-the-counter painkillers.

Scientists have come up with a few ways to "regenerate bone" (make the bone grow again).  Their many ideas for future strategies include doing gene therapy and using 3-D printers.

References

Other websites 
 See illustrations with Simple English explanations of each of the types of fractures  (from Brigham & Women's Hospital)
 Simple English materials on broken bones for children and for teenagers (from KidsHealth.org)
 "Patient-Friendly Bone Basics" about preventing osteoporosis and bone breaks (from the National Osteoporosis Foundation)

 "Classification Of Fracture - Simple And Composite Fracture" First Aid Steps For A Broken Bone(fracturesandbonehealing.blogspot.com) - Simple Bone Fracture Healing Time.

Skeletal system