BMJ Group Medical Reference
The three signs of ADHD are not paying attention, being overactive and acting without thinking. A child can have symptoms of all three. However, to be diagnosed with ADHD a child must have: 
Talking too much and fidgeting, for example, can both be symptoms of hyperactivity.
The symptoms of ADHD are listed in a book called the Diagnostic and Statistical Manual of Mental Disorders. The following list of symptoms is adapted from that book. Not paying attention
Being overactive and acting before thinking
Often pay no attention to detail and make careless mistakes in schoolwork or other activities
Can't concentrate on one game or task for long
Often seem not to listen when spoken to
Often don't follow instructions, and fail to finish schoolwork and tasks around the house
Often have difficulty organising tasks and activities
Often avoid tasks that need a lot of concentration
Often lose things
Are easily distracted
Are often forgetful.
Often fidget or squirm when sitting down
Keep getting up
Often run about or climb instead of sitting still
Often have difficulty playing quietly
Are on the go all the time, and act as if driven by a motor
Talk too much.
But, just because your child has lots, or even all, of the symptoms listed above it doesn't mean he or she has ADHD. Doctors also look for the following.
Your child must have been behaving like this for at least six months.
Some of the symptoms must have been present before your child reached the age of 7 years old.
Above all, your child's behaviour must be causing problems in at least two places (for example, at home and at school). 
Before diagnosing ADHD your doctor will also check that your child's symptoms are not caused by another condition such as bipolar disorder, anxiety disorder, or personality disorder.Types of ADHD
Doctors divide children with ADHD into three groups, depending on how they behave.
Combined type ADHD: This means your child has all three kinds of symptoms. They don't pay attention, can be hyperactive and can be impulsive. This is sometimes called hyperkinetic disorder or HKD. 
Predominantly inattentive type ADHD: This means your child's biggest problem is inattention. Hyperactivity and impulsiveness are not such big problems.
Predominantly hyperactive-impulsive type ADHD: This means your child's biggest problems are being hyperactive and impulsive. Not paying attention is less of a problem.
For references related to Attention deficit hyperactivity disorder click here