What's the difference between sopranist, falsetist and countertenor?

A soprano is the highest natural female singing voice. A countertenor and falsettist are more or less the same thing - a male singer who sings in the alto register by using his falsetto or 'head' voice rather than his chest voice (where he would most likely be a baritone). I have heard a countertenor do a very amusing duet with himself alternating his chest and head voices. Philippe (please note spelling) Jaroussky is that rare animal, a male soprano, which means he can sing right up in the soprano register in his falsetto voice, rather than the alto register like most men.