An illness that is definitely not just a mild form of autism. It is definitely milder than autism, but Asperger syndrome is a different condition and it has different (but similar) symptoms. However, despite all the hype and overdiagnosis, very little is actually known about the condition.
The most commonly-known 'symptom' is having trouble in social situations, including problems making eye contact, for example. Sometimes, people with this condition may have poor motor coordination, which may lead to failure in team sports.
Despite popular opinion, people with Asperger syndrome do not have any trouble realizing social situations or people's emotions. They do, however, have a problem in responding accordingly, mainly because of the fear and tension that human contact causes. This fact is demonstrated by experiments that show that people with Asperger syndrome have no problem identifying the proper social behavior when in a controlled laboratory setting with no people around.
This is counter to people with autism, who do indeed have serious problems diagnosing social situations, not just problems responding to them.
Asperger syndrome causes a huge range of problems in life that can manifest or lead to depression, social anxiety disorder, rejection, and even suicide in some cases.
Despite this, Asperger's is no excuse not to try to lead a normal life, as demonstrated by the large number of people who have succeeded at their jobs, at social life, and marriage despite having Asperger's. However, a person with, say, diabetes may live a relatively normal life too. Just because a disease does not force one to live their life on a hospital bed doesn't mean it is not a real illness.
Despite my having Asperger syndrome and all the problems that go along with it, I've managed to live a relatively normal life using self-discipline, a small but nonetheless friendly social circle, and the occasional dose of professional help/advice.