The Content Search Web Part (CSWP) is a Web Part introduced in SharePoint 2013 that uses various styling options to display dynamic content on SharePoint pages. But when do we use CSWP or the CQWB
First this is a link to introduce the Content Search Web Part (CSWP)
Now to decide any one to use, The CSWP can return any content from the search index. Use it on your SharePoint 2013 sites when you are connecting to a search service and want to return indexed search results in your pages.
The CSWP returns content that is as fresh as the latest crawl of your content, so if you crawl often, the content that the CSWP returns is more up-to-date than if you crawl infrequently. If you need to display instant content or the refreshed version of content, use the Content Query Web Part (CQWP) instead.
Search crawls only the major versions of content, never the minor versions. If you want to display the minor versions of your content, do that by using a CQWP.
Some site collection administrators mark sites to not be indexed. Content marked in this way is not available in a CSWP. If you want to return results from a site that is marked to not index, use the CQWP instead.