Intersex people are individuals who have a combination of what is culturally thought of as male and female anatomy or physiology. In short, an intersex person will have an anatomy or physiology that does not fit into the rigid definitions of male or female.
Some intersex people have genitalia that does not fit exclusively into the constructs of male or female. Other intersex people might have internal reproductive organs that do not match their external sex characteristics. Being intersex is not a disorder, and is a perfectly normal and healthy way to exist.
Intersex is simply a common variation of the human body. In fact, according to The British Association for Counselling and Psychotherapy, there are approximately 358,105 people with intersex variations in the UK alone. Many people may not be aware that they are intersex, since many intersex variations are not external.
Other definitions/things to note:
The biggest thing that intersex people need is support from their family and peers. For intersex children, it is important for them to have support and love from their families, and the opportunity to connect with other children that are also intersex.