i've found this information on the following link (us embassy, Costa Rica -last passage)
would any of our respectable seniors explain it please ?

http://sanjose.usembassy.gov/ivfamily.html

IMPORTANT NOTICE FOR NEWLYWEDS

If at the time of admission to the United States you will have been married for less than two years, you will be granted conditional permanent residence by an officer of the Department of Homeland Security (DHS) at the time of your admission to the United States. You and your spouse will be required to file a joint petition (form I-751) with the USCIS to have the conditional basis of your status removed, once you have been married for more than two years. This joint petition must be filed within 90-days before the expiration date of the conditional permanent residency. If a petition to remove the conditional basis of your status is not filed within this period, your conditional permanent status will be terminated automatically, and you will be subject to deportation from the United States. You will be provided with written information about this conditional status when your visa is approved and issued.