What is the difference between do-follow & no-follow backlinks?

If you’ve been doing any kind of reading about link building, then  you’ve probably seen people mentioning “nofollow” and “dofollow” links.
One of the most important way to achieve high search engine ranking results is to get other blogs or websites to link to your site. If these links are NoFollow then Google doesn’t count them as a link and are the links useless for your search engine rankings.

In a concise way, DoFollow sites are sites that do not use the  nofollow tag.
Links from DoFollow sites are noticed by search  engines, thus boosting their value then the question that quickly comes into ones mind is that

Why  was “nofollow” invented?
In the beginning, all links were dofollow . Search engines (such as  Google) ranked pages based on the number of dofollow (good) links that  pointed to them. Spammers soon learned this and targeted any site  giving away free links  to increase their page’s rank by linking to themselves.
Nofollow simply is a HTML tag added to the link anchor code, and will  change the normal code as:
<a href=”your link URL” rel=”nofollow”>your link name</a>

So to discourage spammers, the nofollow tag was invented to indicate an un-approved link. Nofollow tells the search engine not to follow the link, denying the spammer any increase in page  rank. As of 2009, nearly all blog software marks all comments  as nofollow.

Why is Do-follow Social Bookmarking Always Good compared to other  strategies to gain Do-follow links?
Search engine bots love fresh bookmarks and the links get indexed  faster than in any other strategy.
Do-follow Social Bookmarking is cheap compared to the other  strategies produce the best ONE WAY backlinks.It brings along the benefits of higher search engine visibility and increased flow of targeted traffic as well.
the comments posted with an URL in it on a NoFollow blog are not followed by Google and other search engines. Google initially used the setting NoFollow as an anti-spamming measure.

Offcourse then you will think How can I protect my blog against  spam?
Here are some of the tips/measures that you can use.
you can set a minimum content length on your comments.
Don’t  get  an email for every message, just visit  every couple of days and moderate the comments in bulk. Finally,  make sure your blog holds comments that hold more than 1 link in  the comment body (easiest way  to configure in Wordpress).

How can I tell if a site is dofollow?
Basically, just select “view source” from your browser’s menu to look at the HTML. Look for links that have <a rel=”nofollow”>Joe Commenter</a>. If you see that, the blog is nofollow (bad) not dofollow (good). You can also find Firefox plugins and hacks that  will highlight nofollow links.