During the download process, download managers may not be able to download the whole file. Therefore many download managers will temporarily save the downloaded parts of the file with the file extension .PART, indicating that this represent part of the whole download. Once the download is complete the file is then saved correctly.
A few specific peer-to-peer packages, such as eDonkey and eMule also save partial downloads with this file extension. Files with this extension represent incomplete versions of other, downloaded files. As such there is little point in opening them.