Shy or autistic?

I thought it might be a good idea, for people who are looking for some advice, to explore the difference between being shy and being autistic.

There may be some individuals who suspect that they might be somewhere on the spectrum because they have difficulties with socialising and are not clear about the real differences between simply being timid and having autistic traits.

What are the key differences between someone who feels shy around groups of people and autistic people who have communication issues? How can you tell the difference?