When I search keywords from Google, it shows all articles related to these words and It has separate tab called "News", where Google shows related news.
How Goolge "Knows" that article from site is about News?
I have opened source codes of multiple news websites and they has "itemprops" in their titles and texts. However, keywords inside of these itemprops differ and some sites does not even use them.
Do I need to register my site as "News site" in Google, in order to show my articles in News tab? Is there some kind of SEO trick?
Google will use a number of approaches when building their databases and one cannot say exactly how you would get your site to register within google as a news sight. However you will notice the following meta information within many articles that show up on google news.
<meta property="og:type" content="article" /> <meta name="news_keywords" content="keyword1, keyword2, keyword3..." />
Google also has a document outlining the basics of SEO for websites, that will provide more in depth information.
edit: It turns out that in order to have your site show up consistently on google news you do in fact have to register with google. On top of this you also have to adhere to some fairly basic rules outlined in the site below.