Narrated 'Aisha (the mother of the faithful believers - may Allah swt please with her):

Al-Harith bin Hisham asked Allah's Apostle (peace and blessings of Allah swt be upon him) "O Allah's Apostle! How is the Divine Inspiration revealed to you?" Allah's Apostle (peace and blessings of Allah swt be upon him) replied, "Sometimes it is (revealed) like the ringing of a bell, this form of Inspiration is the hardest of all and then this state passes off after I have grasped what is inspired. Sometimes the Angel comes in the form of a man and talks to me and I grasp whatever he says."

Aisha (may Allah swt please with her) added: Verily I saw the Prophet (peace and blessings of Allah swt be upon him) being inspired Divinely on a very cold day and noticed the sweat dropping from his forehead (as the Inspiration was over).

Bukhari :: Book 1 :: Volume 1 :: Hadith 2