SEO Tips - When to use subdomains

As a professional search engine optimization, I am often respond to questions posed by those lucky few who are new to the SEO field, and among these so-called "newbies" a recurring question often appears up - "Which of the following ways is better URL structure Site? "
www.site.com / subfolder O
subdomain.site.com
And so it is with this recurring dilemma in mind that I will try to answer the question, "that the URL structure is better in terms of SEO, subfolders or subdomains?"

Short answer: if the size of a problem or keyphrase will be discussed in detail so that a new site must be built to house the better the content, or more simply, if the graphical navigation of the site must be changed, use subdomains. In all other cases, use subfolders.

Long answer: To answer this question in detail, we need to go over some basic facts regarding subdomains, subfolders and site themes.

Subdomains:
Represented by the characters in the URL after "http://" and those before. "Site.com"
Are seen by search engines as absolutely unique sites independent from the parent domain.
Authority's website (in other words, the possibility for the site and subsequent pages rank in search engines) are not passed down from parent domain.
They can be listed on the first page of SERP pages along with the main domain. Effectively, all 10 results on the first page of a search engine might be subdomains of a domain single parent.
Subfolders:
Represented by the characters after the URL. "Site.com /" and before "filename.html".
Are seen by search engines as a subdirectory within a site.
Authority Site is handed down from the root domain.
At most, a subfolder can appear on the first SERP on the same domain or subdomain.
The theme of the site:
As a general rule, the more focused the theme of a site easier it is for that site to rank for keywords directly related to its theme.
It is defined using all the pages of the site, not only the best homepage or subpages.
So here's the long answer:

Search engine optimization is all driving the most traffic. Where the objective is to build an authority site with lots of great content that will be found by visitors who use search engines based on laser-targeted search terms, as well as unique "long tail" search terms that Most would never have thought to optimize, but appear in the pages in subfolders of your site and drive traffic to the same.

Using subdomains may appear sexy, because it can lead to a potentially larger number of back-links (though not if each subdomain is hosted on the same IP), or may affect SERPs in your favor by crowding out the top spots with your pages. But without proper promotion, a subdomain only stops potential laser targeted as "long-tail" search by visitors find your site.

My advice is just to use subdomains when the amount of content on a particular topic is so flourishing as to require a separate navigation and design (ie your website), or high enough quality that it will acquire links and be alone.

For example:
If you have a site of 15 pages on your lawn-care service and 10 pages of these revolve around a special fertilizer that you developed, you may consider creating "specialfertilizer.site.com" so you can market your fertilizer separately.
Take my company domain "objectwareinc.com". Recently, we have built up a subdomain "palm.objectwareinc.com", because we had so much information about our solutions for developing wireless applications that only made sense to create a separate site for it.
In this example, the focus is narrow, but there is still a huge amount of content. So we are sending visitors to the subdomain using targeted keywords while visitors are also finding with "long-tail" searches and landing on the pages of palm.objectwareinc.com subfolders.

The general lesson is to create subdomains ONLY when you have the quality and / or the amount of content to support them. Remember that subdomains are independent sites in the eyes of search engines, which means that in order for them to rank, which will require efforts to promote and link building....