When you have simple, fixed layout (i.e. structured) documents, indexing is a simple task of looking at a few locations on a page, and data entering the contents. For simple tasks like this, there are many OCR solutions that can help automate document indexing.
A document input system is the front-end of your document management system. This front-end allows you to classify and index the documents. Some systems extend this to also extract fuller document data for later use.