what is natural link and what is not ? is there any definition from Google if there is then please provide me the answer and also how to get natural link ?
Nope, there is no definition. But unnatural links are links that you buy, gather with blackhat methods or links that try to artificially increase your websites importance. You get a natural link when people link to your site from their site.
As above... If you produce a good quality site with content that users like and enjoy some will share this.. this is natural linking. Buying anything can be seen as unnatural if you do it to much. Just be sure to have a range of anchors if you buy links to avoid it looking totally unnatural and links from various places.
How can u find a site has a good quality of content. everybody is using high quality of content then how u will find a particular site is high quality. is there any criteria about quality content ?
Quality content is content that is written very well (in which ever language the site is targeting), useful to the people visiting the site, on topic and gives information on what people are looking for. Other than the above you can't really say all content on a set of sites is good quality as it depends on the visitors, for example if I went on a site about football I would think the content is rubbish as I don't like football (regardless of how good the content is) but if I visit a site talking about thermochemical equations and the content is well written, on topic I would think of the content as being high quality as I enjoy thermochemical equations.
Unnatural links include but are not limited to: a link from a website that has nothing to do with yours (ex: a link to your car insurance website from a porn site) or little anchor text diversity (ex: 1000 anchors on "car insurance", 9 anchors on "click here", 1 anchor on "insurance")
Natural links contain all the things, click here, know more, nofollow, dofollow, high traffic site, low traffic site, irrelevant sites, relevant sites and so on.
Links built for your visitors are natural links. Links built to manipulate search engines are unnatural links.
Natural links are when users like the article on your site and link to it from their site, or they share it on another site, like on Facebook or a forum. These are the links that positively affect your site's ranking. Unnatural links are usually bought links. Hope that helps. Have a good day!