HTTP response status codes show whether you have successfully completed a particular HTTP request. There are 5 classes of HTTP response status codes:
HTTP header fields provide required information about the request or response, or about the object sent in the message body. With an HTTP request or response, HTTP headers allow the client and server to pass on additional details.
The case-insensitive name of an HTTP header is followed by a colon (:), then its value. Whitespaces before the value are ignored.
Headers can be grouped according to their contexts:
It is the job of search engine crawlers to locate and crawl as many URLs as possible. This done to identify if there is any new content out there.
These URLs may be new ones as well as URLs that they already knew about.
By crawling pages they already knew, new URLs are located. They then pass on their results to the indexer. Search engine pages that are allowed to crawl are also called crawlable pages.