There is a lot of information about apostille in US which can easily be found online. It came to life as the result of The Hague convention of 1961 and then gradually made its way into the document preparation requirements of many countries around the world. This does include the United States.
There are several distinct characteristics of the US Apostille. First of all an apostille in the U.S is a separate page which is attached, stapled or glued to the document, while in the rest of the world it is a stamp placed on the reverse side of the document. Furthermore the apostille in the United States is issued based on the geographical jurisdiction. In other words documents issued in San Antonio, Texas will have to be apostilled in San Antonio and you can hire apostille San Antonio TX service for that, documents issued in California will have to be apostilled in CA and so on.
So why would someone need an Apostille?
It is quite simple. If you plan to use your U.S documents in a foreign country you will need an Apostille. Given your target country accepts them, otherwise you will have to go through a chain authentication procedure known as consular legalization or attestation.
The main function of the Apostille is to validate the capacity and authority of the public official who signed your document, whether it is a notary public or state registrar of vital statistics, you still have to prove that they are who they claim to be to the foreign authorities. And apostille does just that. Without the apostille the piece of paper you present to the foreign official is just that, a piece of paper. Nothing more. For all they care you could have printed that at home the night before.