This article explains the different formats that use the PSP file extension. It also explains how to open and convert the file.
A file with the PSP file extension is most likely a PaintShop Pro image file. Similar to Adobe Photoshop's PSD format, PSP files can store guides, layered images, and other things common to high-end image editing software.
Some PSP files may instead be preference files that store settings for Photoshop. For example, there is a Brushes.psp, Patterns.psp, and Styles.psp file to store settings specific to those features.
PL/SQL server page files that access database information via SQL commands are text files that also use this file extension. Another use for the PSP file extension is as a Pixel Studio project file.