Eventually you will get another I-797C Notice of Action document from the USCIS, but this one is different than all the half a dozen+ I-797C documents you received in the past. The first sentence of the notice will read:
WELCOME TO THE UNITED STATES OF AMERICA ... etc.
It is the approval notice for your I-485, Adjustment of Status Application. You are officially a permanent resident of the United States. But wait, where is your actual Green Card (which is pink btw.)? It does not come with the approval letter. It can take up to 90 days before you get your physical Green Card. You need that card to be able to leave and re-enter the country.
If you have to leave the country after you got the I-485 approval notice, but before you got your physical card, call your local INS office and schedule an appointment to obtain an I-551 stamp (Form I-551), also called "Temporarily Green Card".
It is also required to contact your local DMV office to let them know that your legal status in this country changed from alien worker to permanent resident.
Also remember that your Green Card is not a citizenship. You can't vote and you also can't perform jury duty. You also have to notify authorities via the AR-11 "Alien's Change of Address Card", if you move to a new residence. Your status can be revoked if you convicted of any crimes, leave the country for longer than a year at a time and other more "fuzzy" reasons. It is generally advised to request a re-entry permit from the USCIS, if you plan to leave the United States for longer than 6 months. Better safe than sorry. Good Luck!