Many people who apply for and obtain lawful permanent residency are initially admitted to the U.S. as conditional lawful permanent residents.  The most common instance of the conditional lawful permanent resident arises out of the Marriage Fraud Act which requires that all persons who obtain lawful permanent resident status through marriage to a U.S. Citizen be admitted as conditional residents for a period of 2 years.

Conditional residents are required to file Form I-751 prior to their 2 year anniversary and request the removal of the conditions on their residency.  The adjudication of the I-751 generally takes about 1 year.  When an I-751 applicant's petition is accepted by the USCIS, the receipt notice that is issued provides that employment authorization and travel authorization are extended for 1 year. 

Although the receipt notice indicates that travel is authorized for a period of 1 year, the document can only be used to return the U.S. after travel abroad within 6 months of its issue date.  Thus, if you plan to travel while your I-751 is pending and won't return within 6 months of the date the receipt notice is issued than you should apply for a temporary I-551.