Parce que is a subordinating conjunction and can begin a sentence. Parce que introduces a cause, explanation, or motive - it explains why something is done.

Je ne suis pas venu parce que mon fils est malade.

I didn't come because my son is sick.

Puisque is a subordinating conjunction and can begin a sentence. Puisque gives an obvious explanation or justification, rather than a cause.

Tu peux partir puisque tu es malade.

You can leave, since you're sick.

source: http://french.about.com/od/grammar/a/conclusions.htm