Oxford Concise: One who holds irrespective of reason, and attaches disproportionate weight to some creed or view. - Originated in the 16th Centuary
In other words, a bigot is someone who's intolerant and prejudice, whether it be to another person's quality (gay, jewish, foreign, etc.) or to their way of thinking.
He/she is a bigot
He/she is performing an act of bigotry