Configuring Content Sources
When you create a Content Source, you can specify the kind of thing you want SharePoint to crawl.
Each Content Source has a type, and can be one of:
User Profile Content
Exchange Public Folders
Custom – which requires 3rd party or custom developed code.
You can have multiple paths per Content Source.
You can also specify a Full Crawl and Incremental Crawl schedule.
For SharePoint-based Content Sources only, you can enable Continuous Crawl.
This one checkbox allows SharePoint to launch overlapping Incremental Crawls – by default 15 minutes apart.
You can bring this down to 5 minutes apart using PowerShell.
The smaller the interval, the fresher the content in your index.