Web server diagnostics offers the following types of logs that can be enabled and disabled:

• Web server logging: This type of log provides information using the W3C extended log file format to log information about HTTP transactions. This is useful for determining the overall site metrics, such as how many requests are coming from a specific IP address and the number of requests that are handled.
• Detailed error logging: This type of log provides detailed information for any request that results in an HTTP status code of 400 or greater. This log contains information that can help you investigate why the server returned the error code. For each error in the app's filesystem, one HTML file is generated. 
• Failed request tracing: This provides detailed information on failed requests. This includes the IIS components that were used to process the request and the time each component took. This is useful for improving the site's performance and isolating a specific HTTP error. One folder is generated for each error in the app's filesystem.

For web server logging, you need to select a storage account or filesystem. When selecting a storage account, you need to have a storage account in place with a blob container. You can also create a new one. If you store the logs on a filesystem, they can be downloaded as a ZIP file or accessed by FTP.