In SharePoint terms, what's the difference between indexing and crawling?
Often, people use those terms interchangeably, as in many situations the differences don't matter. But imagine a school nurse needs to get forms from all new students. So the school nurse gets a student volunteer to go to each class and bring any new student to the nurses office. In this case, the student volunteer is the crawler, while the nurse who is recording the data is acting as the indexer.
The crawler makes requests for content and is in charge of determine which files should be processed. The indexer is responsible for building the index by processing the files returned by the crawler.
In the SharePoint search service:
Crawling: Is the mechanism that scans all pages, documents, ListItems, etc. contained in SharePoint. With this scan you can get contents.
Indexing is the mechanism that integrates the content in the search database (you can imagine a link between content and its URL). And after, when you execute a search query, you will query the indexed content and the search will return the URL of this content.
This is a general explanation ;)