What is a robots.txt? This website has several articles about SEO and robots.txt.
Robots.txt is a text file in ASCII for declaring the web page not to be searched by the search robots (or search spider). Although not many search engine robots support the Robot meta tag, some famous search engine such as GOOGLE can completely support, and even add a comment of "archive" to limit if the GOOGLE can keep the screen shot of the web page.
＜meta name="googlebot" content="index,follow,noarchive"／＞
Location: /robots.txt (root directory)
Format: example from google
User-agent: robot-name (can see the Web Robots list for details)
Meta-tag (can be 1 of 4):
＜meta name="robots" content="INDEX, FOLLOW"／＞ , can be written as ＜meta name="robots" content="ALL"／＞
＜meta name="robots" content="NOINDEX, FOLLOW"／＞
＜meta name="robots" content="INDEX, NOFOLLOW"／＞
＜meta name="robots" content="NOINDEX, NOFOLLOW"／＞
INDEX is for declaring to index your page in search engine, while NOINDEX is not.
FOLLOW is for declaring to index the links from your page in search engine, while NOFOLLOW is not.
Links is very important for Internet, so many search engines always see links (mainly in-links) as important factors for indexing. Beacuase of this, many sites improve their SEO by take the convenience of exchange links. However, despite of of common of exchange links, google has penalty of this behaviour due to decrease the quality of the links.
Therefore, we should not use the method of exchange to increase the number of in-links of the website.
Other useful links for SEO: