http://www.mayoclinic.org/diseases-conditions/autism-spectrum-disorder/symptoms-causes/syc-20352928
Its a spectrum of what you could call personality traits that range from quirky to debilitating. In some ways everyone could be considered autistic in some regards, but generally someone who has an Autism spectrum disorder will exhibit social/personal abnormalities that exceed your average "awkward" person. For me it meant that I had a really rough time growing up, I was never really able to have close friends or relationships outside of family and a few select peers. But as I got older I kinda "grew out" of the more debilitating aspects, and now its more just day to day communication challenges.