I don't want the search engine to go through some of my pages of my blog, can someone teach me how I can add nofollow tag to those pages or use robot.txt to block the pages? What is the difference between two?
none. rel="nofollow" added: <a href="http://www.somesite.com/" rel="nofollow" >blah</a> with robots.txt you can disallow the spidering / follow of local links. with rel="nofollow" you can apply it to remote links also, preventing the "bleeding" of page rank. as for adding it, the best practice here dictates, in addition to all eternal links, you evaluate your core keywords/content and apply to anything that won't rank very well there. for example, google does not need to know your contact us, terms and conditions, faqs etc - these kind of pages have no seo value, especially if you are trying to promote numerous categories, products/articles, search tags and so forth. on dynamic menu/cms systems, you can do a simple change to your Db that has nofollow set("true", "false") not null default "false". then as you read data and build links based upon it, compare nofollow and if true, add rel="nofollow" to the link.