The most commonly used records in the National Archives are the census, military Records, and immigration records, along with ship passenger lists, naturalization, and land Records.
But if you think all of these records are accessible by online search, you will be disappointed. In fact, the National Archives web site has very few records online. Mostly what can be found are indexes and finding aides. The exceptions are a bit of obscure records of history, which may help a few genealogists and are listed on the web site at National Archives And Records Administration
Many of the documents have already been digitized by Ancestry Library and Heritage Quest. Ancestry Library is part of Ancestry.com and does not lend copies for home use. Ancestry allow libraries to subscribe to these digital records, and one must check the local library to see if it has access. Heritage Quest CDs are available directly to the individual or through some libraries. My local genealogical society has a few of the CDs in their collection. You can also access National Archive records directly at the main library in Washington D.C., and certain collections are available at various regional branches in the United States. Remember, viewing of such records can be arduous and take many hours, so plan to stay awhile. And if you have more money than time on your hands, you can always hire a researcher to go in your place.
Census Records (1790 to 1930)
It is funny what little tidbits you can pick up, like how often had you heard that you can only access census records which are not later than 1930? Did you know the official policy is a "72" year restriction on access to population census schedules, which is why 1930 is the last available record?
Immigration (ship passenger lists)
The National Archives has arrival records between 1820 and 1982 and are organized by port of arrival. The Archives suggest that records prior to 1820 might be found at state archives where the port in located.
There are over 10 million land records at the Archive, which are transfers of public lands from the government to private ownership.
Military Service and Pension (1775 -1902)
The archives in Washington D.C. hold Federal military records from the Revolutionary War to 1912. World War I records are at the records center in St. Louis, Missouri. Most genealogists request a compiled military record of a particular person. These include, service records, pension applications and bounty land records based on claims of wartime service.You should realize that you are going to need to know your ancestor's branch of service, the conflict he fought in and whether he was in the Regular army, a volunteer unit or was an officer.
I hope this explains what can be obtained by going to the National Archive's web site and or at its record centers. I personally learned some things about how those records that are available are not all that easily accessible. If you do find a record by one of the various methods mentioned above, you can purchase a copy of the document (or microfilm) for your own use. We all would like everything to be at our finger tips, and perhaps some day all the information will be accessible. But for now, there is no replacement for road trips, despite all the wonderful advances in computer technology.