Ok so my question is which do we need to file or do we file both forms. I am confused about which we need.
When your home study is complete you file the I-800a. This form is your application to the USCIS to allow your family to bring an orphan into the US. Any orphan, period.
After you have reached a certain point in your process (not sure what country you are applying for) you file the I-800 which is your application asking USCIS for permission to bring a specific child into the US.
I-800a: general permision for any orphan
I-800: permission for a specific child.
You must file the I-800a first, get approval and then you can file the I-800 at the appropriate time in your process.
In Hague adoptions that do not involve adoption of a known child, such as a relative, you submit the I-800A first, before a child is identified. You generally file it with the USCIS as soon as you have your homestudy report in hand, since you must include the homestudy report with the submission.
The I-800A allows the USCIS to make a preliminary determination of the prospective parents' fitness to bring an orphan into the United States. The USCIS looks at the application, the attached homestudy report, and the other requested documents in making the determination. In this respect, the I-800A is a lot like the I-600A, which is used in non-Hague adoptions.
If and when your I-800A is approved, you will include a copy of the approval form you get from the USCIS (usually called a 797-C or 171-H) with the dossier you submit to the foreign country's "Central Authority" under the Hague. It lets the foreign country know that the U.S. government considers you fit to adopt a child and bring him/her to the U.S. Hague countries will not refer you a child unless they have such an approval in hand.
Once you have received, reviewed, and accepted a referral from the foreign country, you will complete the I-800. The I-800 is a companion document to the I-800A you submitted earlier. You will submit it to the USCIS with various documents about the child whose referral you have accepted. It allows the USCIS to look at the CHILD'S paperwork, and make a preliminary determination as to whether the child meets the requirements for a Hague adoption.
If the child's paperwork is in order, the USCIS will send your provisionally approved I-800 and the supporting documents to the U.S. Embassy in the foreign country. The Embassy will also need to give provisional approval of the I-800 -- primarily looking at whether the child meets U.S. requirements for an adoption visa -- and notify the foreign country that it is OK to allow the adoption to proceed. The form that the Embassy issues to the foreign country is usually called an Article 5 letter.
Once you complete the adoption of your child (or gain legal guardianship of him/her in the case of countries that let you finalize in the U.S.), you will still need to get a visa for him/her to enter the U.S. With most countries, this will occur when you are actually in the country. You will go to the U.S. Embassy, generally after a visit to a clinic for a mandatory physical exam of the child, and the Embassy personnel will do one last review of your I-800 application and all documents about your qualifications and the child's qualifications. If all is in order, you will be granted a visa for your child and can bring him/her home.
The I-800 process is a bit different from the I-600 process, in that part of the approval process customarily takes place in the U.S., before you travel and adopt your child. With non-Hague adoptions, using the I-600, you don't generally complete the I-600 until you get to the foreign country and take custody of your child. While there are some extra steps to the I-800 process, which you may resent because they take time, be aware that it is a heck of a lot better to know BEFORE you travel that there is almost a 100% certainty that the child you adopt will qualify for a visa. Ask any non-Hague family that has faced visa problems in countries like Vietnam and Guatemala. Some of these families traveled, took custody of their children with the legal and moral responsibility that such custody entailed, and then found that the U.S. Embassy was going to deny their children visas because of perceived irregularities.
So the short answer is that, in most Hague adoptions, you submit both an I-800A and an I-800. The I-800 is submitted before a child is referred to you, and the I-800 is submitted after you have accepted a referral, but before you complete the adoption.