I have some questions about search engine optimization. I am confused that whether i should add title to the link on the website such as <a href="http://***" title="China professional DVB manufacturer">China professional DVB manufacturer</a> ?
And the second question is: the comments added to the website is useful ?
Thanks in advance

Use the title attribute to deliver additional information to the person just about to click the link. The text should appear as a pop-up tool-tip on mouseover.

The title attribute is for accessibility issues. The text from the title attribute is not (currently) indexed or ranked by any of the (major) search engines.

You can use title for link since it helps the user experience but please keep in mind that you do not write too much content in the title (DO NOT spam SEs). Other things like adding summary to tables can also be used but as per my experimentations, these tags have no effect on rankings.

Comments are not indexed (Again, can be used to potentially spam SEs, therefore SEs do not even bother). You should no bother either. Hope this helps