How OCR works
OCR, or optical character recognition, is programmed to recognize specific outlines within scanned image files and identify them as text. Based on the individual shapes of different letters, numbers, and symbols, OCR software is able to convert entire documents from images into fully searchable and editable text. OCR works only with printed text, though other types of software can handle handwriting as well.
Features of most free online OCR tools
When using most online OCR tools, you can upload a file to the website, and receive an OCRed file as output. Most of these programs work reasonably well, but their accuracy is far from perfect, the process is slow, and they are somewhat limited in their capabilities. There are also some free programs available for download, but you may run into some of the same problems with these.
A better OCR option
Instead of trying to deal with these tools, consider starting with a professional OCR program. Even if you do not want to immediately invest in a costly product, some are available for free. For example, Maestro Recognition Server from CVISION is available in a free trial version that you can download without any risk. Begin using it now to see if it is the right solution for you.