As a permanent resident or conditional permanent resident you can travel outside the United States for up to 6 months without losing your green card.
Staying outside the United States for more than 6 months but less than one year will subject you to additional questioning when you return to the United States but you are not required to have a Reentry Permit and shouldn't have any problems getting back in.
If you intend to stay outside the United States for a year or more you will need a Reentry Permit. You apply for the Reentry Permit on Form I-131 Application for Travel Document. You can't apply for a Reentry Permit while outside the U.S. so make sure you get your I-131 submitted before you leave the country. The Reentry Permit is good for up to 2 years and may not be extended. If you are a conditional permanent resident your reentry permit will expire on the same date you are required to apply for removal of the conditions.
If you do not obtain a Reentry Permit and stay outside the United States for more than one year the USCIS may determine that you have abandoned your permanent or conditional residence status.
Staying outside the United States for a year or more will start the clock over for naturalization. This means that the normal 3-5 year wait to become a naturalized citizen will start over if you are out of the county for more than a year. However you can file a form N-470 Application to Preserve Residence for Naturalization Purposes the approval of which will preserve the "clock" for naturalization purposes.
