It is putting too many of the keywords into a page. Google does not like it and it has no value on your web page if you do it.
That's why there are some rules to put url in the pages,like if you are going through Web 2.0 there you can put limited keyword,same for meta tag,title tag,description etc....
These are few mistakes which commonaly done by most of the SeO professional:-
1-No Keyword Research.
3-Building the Wrong Links.
4-Not Leveraging Social Media.
5-Poorly Developed Websit
On page seo is main part of seo. what you think
I heard about On page and off page optimization in SEO,but I have no idea about these,So could you please tell me brief about on and off page optimization?