A falsetto voice is a special way of speaking or singing. If a man tries to imitate a woman’s voice he does it by speaking in a falsetto voice. When a man sings with a falsetto voice it sounds high like a soprano. Actors do this sometimes, e.g. Robin Williams in the film Mrs Doubtfire or Kevin Clash when voicing the character Elmo in Sesame Street and Barry Gibb of the Bee Gees, with his R&B falsetto. Also, Kellin Quinn from the band sleeping With Sirens uses falsetto sometimes.
Women can also use falsetto, but when they do there is not as big of a change in sound from their normal voices as there is in men. When women use falsetto the voice sounds lighter or weaker than it does when they speak or sing normally.
It is like stretching the mouth out into a wide smile and saying 'ee', except it is sung.