CodeSniffer is a PHP package that checks code against a defined coding standard to ensure consistency. It can sniff any PHP file or directory and produce a detailed report of errors and warnings found. Common standards include PEAR, Squiz, PHPCS, and Zend. The document discusses how to install and use CodeSniffer via the command line, generating reports, and running it multiple times to catch all issues. Custom standards can also be created by combining rules from different standards.