DPI and CPI describe similar ideas
DPI means dots per inch, while CPI means counts per inch. A mouse sensor reports counts rather than printing physical dots, so CPI is often considered the more literal technical term. In consumer mouse software, however, DPI is the overwhelmingly common label.
Why manufacturers say DPI
DPI has a long history in pointing devices and printing, so it is familiar to users. When a manufacturer lists 800, 1600 or 3200 DPI, it is generally describing the mouse's configured counts-per-inch behavior.
Does CPI change how the mouse feels?
The number itself is a setting; what you feel depends on the complete input chain. If you double DPI and halve an appropriate game sensitivity, the resulting in-game sensitivity can be similar.
DPI/CPI and polling rate are different
DPI concerns counts per physical inch. Polling rate concerns how frequently the mouse reports to the computer. A 1000 Hz mouse does not automatically have 1000 DPI, and changing one does not necessarily change the other.
How to compare two settings
Use eDPI for same-game comparisons and cm/360 when you want a physical movement measure. Use the DPI Analyzer to independently check pointer behavior.