There are different display templates for all types of results. You have display templates for word documents, web pages, links, images, etc.. etc…
The bold text is whatever the SharePoint Search Index finds relevant to your search. The summary of the item could in fact from from the body of the document or elsewhere. It is important to note that these display templates can change if you want to include other parts of an item in your search results. You are not limited to the default.
Can you answer these?