The International Digital Publishing Forum (IDPF) is the trade and standards association for the digital publishing industry. See http://www.idpf.org/. Their logo looks like <idpf>
They have defined a number of acronyms including .epub, OPS/OCF & OEB which are defined in the following quote from the site.
"'.epub' is the file extension of an XML format for reflowable digital books and publications. '.epub' is composed of three open standards, the Open Publication Structure (OPS), Open Packaging Format (OPF) and Open Container Format (OCF), produced by the IDPF. '.epub' allows publishers to produce and send a single digital publication file through distribution and offers consumers interoperability between software/hardware for unencrypted reflowable digital books and other publications. The Open eBook Publication Structure or 'OEB', originally produced in 1999, is the precursor to OPS."