A K-1 (fiancé) visa is a temporary visa for couples who are not yet married to their U.S. citizen partner. As a U.S. citizen, you may petition your foreign fiancé to come to the United States to get married within 90 days of entry. Once married, the foreign spouse can adjust his or her status.
On the other hand, a CR-1 (spousal) immigrant visa is for spouses of U.S. citizens who want to immigrate to the United States. Unlike the K-1 visa, the couple is already married.
A comparison chart between the Fiancé and a Spousal Immigrant Visa helps illustrate the differences: