Content Index: For SharePoint to search any content, the content must first be indexed. A content index combines details on all the information in the content sources. When users perform searches, the index is queried for content that matches the user-entered terms.
Crawl: For a search to be accurate and effective, the system updates the index regularly based on a schedule that you define. A crawl is the process by which the SharePoint index is rebuilt or updated to include new information:
a. A full update is a complete recrawl of all SharePoint content to update the index.
b. An incremental update only reviews items that have changed or been added since the last update.
iFilter: SharePoint crawls a number of file types to include content in the documents, the system does not automatically crawl some file types and requires you to install a special tool, called an iFilter, to allow the SharePoint search service to index the documents. For example, you must install an iFilter to support PDF document indexing.
Content Scope: Searching the entire content index or a content source may be too broad of a search. By way of example, if we lose our ID card visiting the front lobby of a office and go back to search for them, we limit your search to that lobby, not the entire building. In essence, scopes tell SharePoint what sections of an index to search. We may have scopes that represent smaller subsets of data such as user profiles or the human resources site, that allow users to limit their searches.
Managed Properties: We can configure site column as managed properties so that users can search for documents based on specific content in the Advanced Search interface. For example, if we have a customer site column, we can turn it into a managed property, and when users conduct a search from the Advanced Search interface, they can select Customer from a drop-down list. This refines the search and limits it to items relating to a specific customer.
Attempt a Question on SharePoint Search: Question on SharePoint Search