This question was posted on the Yahoo! answers website and the responses posted by ignorant Internet users were typical of the anti-Ahmadiyya propaganda. I posted this as an answer:
You can learn about Ahmadiyya Islam at their official website http://www.alislam.org
They believe that the Messiah and Mahdi being awaited by Muslims has arrived in the person of Mirza Ghulam Ahmad, to spread the true peaceful message of Islam to the corners of the world. They have missions all around the world. Their members contribute and donate to spread the message and build mosques. They have just built the biggest mosque in Canada and are now building one in Ireland. They built the first mosque in Spain after 700 years. They run 24/7 TV channel called Muslim TV Ahmadiyya (http://www.mta.tv) again run by the donations of their members (unlike oil money of Arabs training terrorists in Afghanistan and Pakistan)
They have translated Holy Quran (yes of course same Holy Quran that was revealed to Holy Prophet) into 70 languages of the world and have established their missions in 190 countries.
Rest of the Muslims are still waiting for the Messiah/Mahdi.
That is only difference. Rest is all propaganda and lies by detractors.