If you are abroad and wish to immigrate to the US, you need to go through consular processing. Whereas if you are already in the US and are eligible to apply for a green card based on being sponsored by an employer to family member or based on holding asylee or refugee status, you can file an Application to Adjust Status to Permanent Resident through Form I-485.
Adjustment of status :
Simply put, if you are currently in the U.S. and have an approved Immigrant Petition, you will be eligible to file an Application to Adjust Status to Permanent Resident of the US. Unless you are applying in a category for which visa numbers are always available, you must have a “current” Priority Date in order to be eligible to file. If you are applying based on your relationship with a U.S. citizen spouse, the parent or child may be eligible to file the Application for Adjustment of Status to Permanent Resident at the same time that the Immigrant Petition is submitted.
If you are an individual who held asylee or refugee status for one year or more, you may also be eligible to file to adjust status to Permanent Resident. If you are outside the U.S., you will not be eligible to file to adjust Status to Permanent Resident. In such a case, you can get a green card status by applying for an Immigrant Visa at a U.S. consulate in your country. The I 485 green card form can also be used by certain Cuban nationals to request a change in the date that their permanent residence began in the US.
You need to file I 485 for green card with the needed supporting documents and fees with the USCIS. If you are 79 years of age or older, you will not be charged a biometric fee. If you are filing Form I 485 on the basis of being admitted to the United States as a refugee, then you need not pay any fee.
After you send the application to the USCIS, and while it is being processed, you can travel outside the United Stated. But you need to get advance permission called Advance Parole to return to the U.S. In case you do not apply for Advance Parole before you leave the United States, you will abandon your application with United States Citizenship and Immigration Services (USCIS), and you will not be allowed to return to the United States.
If your Form I 485 green card application is denied, USCIS will send you a letter that will tell you why the application was denied. Currently, if you are not in a legal status in the US, the process to remove you from the country will begin as soon as your application is denied. In such a case, you can have an immigration judge review the denial of your application during removal proceedings. During this review, Immigration officials should prove that the facts on your I 485 application were untruthful and that your application was rightly denied. After this, if the immigration judge decides to remove you from the country, you can still appeal this decision. Normally, you can appeal within 33 days after the immigration judge decided to remove you from the country. The appeal will then be referred to the Board of Immigration Appeals once your appeal form and the required fee are processed.