One major difference between these two crimes is that extortion requires (to some degree) the consent of the victim whereas robbery does not.
In both crimes, money or other valuable things are taken from a victim by force or by threat. However, in a robbery, the victim does not consent to give the valuables up. The robber takes the valuables from the victim by force. In an extortion, the victim does consent to give the valuables to the criminal. The victim does not do so willingly, but he or she does decide to give up the valuables rather than be subjected to the violence or the humiliation or whatever is threatened.
Another difference is the nature of the threat. In a robbery, the threat is of immediate physical harm. In an extortion, the threat is more distant and may not actually involve physical harm at all.
Here is a quote that nicely sums up the difference
Robbery differs from extortion in that the property is taken against the will and without the consent of the victim, unlike extortion, where the victim consents, although unwillingly, to surrender money or property. Another distinguishing factor is that the nature of the threat for robbery is limited to immediate physical harm to the victim or his or her home. Extortion, on the other hand, encompasses a greater variety of threats.