They are complimentary in many ways and there are areas of overlap. Ambition is fired by desire. If my ambition is to become the Prime Minister, I am motivated by desire for the perceived benefits of being a Prime Minister – fame, power, authority, etc.
At the same time, desire is born out of internal impulses of pleasure and gratification. These may or may not be within the realms of possibility. For instance, a short person may desire to be tall. A person with physical inadequacies may desire to be a glamorous superstar. A penniless, illiterate may desire to be King and have all the wealth, glamour and power it would bring.
Ambition on the other hand is desire tempered by reality. Desire becomes ambition when there is a possibility of acquiring the desired goal. Like an executive who has the ambition of becoming a CEO, or the director of his own business house.
Both desire and ambition, when tapped in the right way can be great motivators. Both trigger passion, and because of it, energy and effort.
Ambition often requires identifying the right desires that will contribute to the achievement of the goal and putting aside desires that may divert one from the goal. For instance, the executive who has the ambition to become a CEO cannot afford to give in to the desires for pleasure like frequent partying, drinking etc. He may desire an attractive colleague of the opposite sex in his office but will put aside this desire to project a professional image. He may sacrifice the desire for weekend bashes to acquire a higher qualification. In this he is deferring the gratification of certain immediate desires for achieving the long-term desire for growth, power, and wealth.
Both ambition and desire have positive as well as negative possibilities. Desire can often lead to overindulgence and ineffectiveness. Ambition without sensitivity and values can lead to manipulation and cruelty. Tempered by sensitivity and wisdom, both can be very powerful and complimentary motivators to great achievements, success, and happiness.