I do not understand the difference between autism and autism spectrum disorder?

Interchangeable: There's no difference between those terms. Spectrum Disorder is simply a newer classification term referring to a family of disorders. The same is implied by saying someone simply has "autism." You are not specifying which behavioral problem they have....just generally saying there is a particular behavior/personality disorder which is recognized with this GROUP of problems.

Autistic symptoms: manifest to varying degrees, Mild (sometimes called "high functioning" is at one end of the spectrum and more pronounced symptoms at the other end w/lots of folks somewhere in between. One view of where autism comes from can be seen at the great plains laboratory website. They look at it from a metabolic point of view and test for it. Peace and good health.

Lump vs. split: Some people like to lump things together into similar categories (this time funding sources). I like to use the analogy of abdominal pain, some would have appendicitis (specific=autism) while others may just have lots of vague symptoms that are not specific to appendicitis (spectrum). With ASD, however, the issues are long term and quite significant.