Issuu (pronounced like "issue"; do you want to make an issue of it?) is an electronic publishing platform. Its format, whatever it may be, is found on the Issuu server, where it's served through web pages with an embedded reader that adapts to platforms, devices, and screen formats at the user's end.
The creator of content published in this platform creates the work (a newsletter, for instance) in one of the supported import formats; PDF is preferred, but MS Word DOC and DOCX, PowerPoint PPT and PPTX, OpenOffice, WordPerfect, RTF, and StarOffice SXI formats are also accepted. This gets converted to the Issuu format on their server.