Christianity Today conducted a fascinating study of "Christians" in America about a year ago. Their research found 5 distinct groups summarized as follows (Link to whole article):
Active Christians 19%
Believe salvation comes through Jesus Christ
Committed churchgoers
Bible readers
Accept leadership positions
Invest in personal faith development through the church
Feel obligated to share faith; 79% do so.
Professing Christians 20%
Believe salvation comes through Jesus Christ
Focus on personal relationship with God and Jesus
Similar beliefs to Active Christians, different actions
Less involved in church, both [...]