The first six characters identify the issuer, the next two the specific issue, and the ninth is a check digit. Different share-class lines, bonds, and post-reorganization-equity each get their own CUSIP, which is why a CUSIP change is a reliable signal that the security itself changed rather than just its name.
Corporate actions are processed by CUSIP. When a broker reports a stock-split, a spin-off, or a merger exchange, the CUSIP on the statement tells you exactly which line was affected.
Example: a company's common carries one CUSIP and its Class B another. After a merger the surviving entity issues a new CUSIP; positions in the old one are converted and the old identifier is retired.
Related: isin, ticker-symbol, sedol, corporate-action, transfer-agent