ADF: The most common disk image format used by emulators. An ADF (Amiga Disk File) is a virtual copy of a real Amiga floppy disk and is normally 880kb (901120 bytes) in size. ADFs do not support copy protection. See the Wikipedia entry for more info.
IPF: A proprietary disk format, created by the Software Preservation Society (SPS - formerly known as CAPS). An IPF (Interchangeable Preservation Format) file is an exact replica of an original floppy disk including copy protection and custom disk formats. This format was created so all floppy games can be preserved before the original disks become unreadable (lost for good). See Software Preservation Society website - Knowledge Base page for more info.
You can add IPF support to your Windows, Mac or Linux Amiga emulator by downloading the user library and following the instructions you find there.
Note that the SPS project's aim is not to promote the distribution of copyrighted software, but to preserve software that may otherwise be lost. The English Amiga Board supports the SPS and so does NOT condone or encourage the distribution of IPF images. Please do not ask for IPF images on the EAB forums, or provide links to websites hosting them.