Dynamics 365 includes several search options, Quick Search, Global Search (also called Categorized Search) and Advanced Find.
In version 8.2 of Dynamics 365 Online a new search method called Relevance Search was introduced.
This searches multiple entities and offers many advantages over categorized search. It uses Azure services.
I won't provide a detailed description of relevance search, because these Microsoft articles do a good job. This article, https://technet.microsoft.com/library/mt723654(CRM.8).aspx describes how to set up relevance search and this article, https://www.microsoft.com/en-us/dynamics/crm-customer-center/use-relevance-search-for-faster-comprehensive-search-results.aspx describes how to use it.
Here I'll provide a high-level summary of enabling and configuring relevance search.
Enable Relevance Search
Relevance Search is enabled in System Settings on the General tab
Choose Entities searched
The entities searched by a relevance search are selected in a solution and is a different set from the entities searched using categorized search.
Use the default solution to select entities for relevance search because then all entities are available for selection. In any other solution, only the entities in the solution can be selected
In the solution, navigate to the entities node and then on the right-hand side, click Configure Relevance Search.
In the Select Entities window, select the entities to include. There is no limit to the number of entities that can be selected but there is a limit to the number of fields that can be indexed. The total used is shown on this window - but the total only includes the entities present in the solution.
Enable Entity for Relevance Search
An entity can only be selected for relevance search if the Managed Property, Can enable sync to external search index is set to True.
Quick Find View
The View columns selected in the Quick Find View are used to display the results of a search. Only the first four columns are used. Any additional columns in the view (that are option sets) can be selected by a user in Facets and Filters in Personal Options for further filtering of results.
The Find columns selected in the Quick Find View are used for searching. Not all column types are valid for relevance searching. You are able to select these columns because the Find columns are also used by Quick Search and Categorized Search.
The Quick Find view criteria are applied in filtering results.
Fields from related entities are ignored.