The K-1 visa is for foreign nationals that are engaged to be married to a U.S. citizen. The purpose of the visa is for the foreign future spouse to temporarily enter the U.S. Thereafter, the couple has 90 days to get married and apply for permanent residency. Failure to do so, will result in the removal of the foreign fiancé.
K-3 Visas are for Spouses of U.S. citizens whom want to enter into the U.S. under non-immigrant immigrant status, while waiting for approval of an immigrant visa or I-130 (Petition for Alien Relative). The process allows the Non-U.S. citizen spouse to enter the U.S. quicker.
Conditional Permanent Resident - receives a green card valid for 2 years. In order to remain a permanent resident, a conditional permanent resident must file a petition, form I-751 to remove the condition during the 90 days before the card expires. The conditional card cannot be renewed. The conditions must be removed or you will lose your permanent resident status.