Words like Ardor and Ardour are very easy to come by in English language. The truth about these two words is that they are pronounced the same way, they have the same origin. The first difference you will notice is the presence of letter 'U' in one while the second word does not have it. The problem with the two words is that, the one with 'U' is not acceptable in American English but the one without is acceptable, also, the one without 'u' is not acceptable in British English, while the second is acceptable.
There are a lot of words like that which are only different from one another by the presence or omission of letter 'U'. Examples are odour and odor, color and colour, etc. Ardor or ardour means great warmth of feeling; passion. It also goes with synonyms like fervor which means an intense, heated emotion. Apart from the little difference between the two words, they are still the same