Adobe Reader is a free-licensed program created and distributed by Adobe Systems. It is used to open PDF documents. PDFs can be a wide variety of files, such as images, text documents, forms, books, or any combination of these. They are cross-platform, meaning each PDF will look the same on a Windows computer as it will on a Mac.
Adobe Reader cannot create PDFs it can only open them. If you would like to create or edit a PDF, you will want to get Adobe Acrobat, which unlike Adobe Reader, is not free.