There are two main reasons why people get headaches, and doctors therefore classify headaches into two broad categories: primary and secondary.

Primary Headaches
Primary headaches are not associated with an underlying condition. However, these common headaches are legitimate biological disorders and not psychological conditions. Some of the common primary headaches include:  

• Tension Headaches:  These are the most common forms of headaches, with approximately 78% of adults experiencing these during their lifetime.Unlike migraines they are not throbbing but band-like and aching, and typically are not severe. Women are twice as likely to suffer from tension-type headaches as men. Causes can include:
• Muscle tightening in the back of head and scalp 
• Poor posture
• Inadequate rest
• Emotional/mental stress or depression      
• Migraine Headaches: 12% of the population experience a migraine. These headaches are more severe, throbbing, and can be felt on one side. Migraine headaches can be caused by reduced blood flow to various areas of the cerebral cortex. Once an attack begins, the pain and other symptoms of migraine arise from an inflammatory process resulting from an interaction between the trigeminal nerve and blood vessels in the coverings of the brain.  Causes include:
• Emotional stress
• Sensitivity to chemicals and preservatives in food
• Caffeine
• Fatigue
• Changing weather conditions
  Article: A Natural Approach to Migraines  
• Cluster Headaches:  Clister headaches are rare and affect 1 in a 1,000 people. About 90% of those that experience these headaches are male, and they can be the most severe of all the headaches. Similar to the migraine, the pain is related to the inflammatory process resulting from an interaction between the trigeminal nerve and blood vessels in the coverings of the brain. Attackss typically recur daily or almost daily for a period of usually 4 to 8 weeks. The headache increases in intensity within 10 to 15 minutes and lasts for 30 to 45 minutes. Causes can include:
• Seasonal during spring and autumn 
• Excessive smoking and drinking
• Sleep apnea 

Note: Primary headaches can also be caused by too much blood flow. For example, caffeine reduces blood flow in the brain. Some pain relievers contain caffeine to take advantage of this effect. If you have been taking caffeine every day and you stop, you can get a headache because of the increased blood flow in your brain.

Secondary Headaches
Secondary headaches are associated with medical conditions like infections, fever, head injury, hypoglycemia, tumors, dental conditions or increased pressure in the skull and/or sinuses (sinus headaches). They result from some other problem with your body. For example, some may get a headache when they have the flu or a cold. Once you eliminate the sickness, you eliminate the secondary headaches.

Common Environmental Triggers

• Stress
• Pollution
• Noise
• Lighting
• Weather changes
• Odors

Common Dietary Triggers

• Nitrates and MSG
• Alcohol (red wine, champagne)
• Caffeine (coffee, tea, cola)
• Chocolate
• Medications