Subdomains – Benefits & Drawbacks
A subdomain as the name suggests is a sub or second-level of a domain. Subdomains do not take on the "WWW". All subs start with "http://subdomain/maindomain.com.
Subdomains are just like main domain with the only difference being it does not accept www. Subdomains do not affect the rankings provided they are worked on considering as a new site.
Subdomains are crucial from the search engine point of view. They prove to be a source of one-way links. A site with many categories is penalized while submitting to the directories. This can be avoided by submitting each category as a subdomain and the links from the subdomain to the main domain will serve as one way links from another domain.
The search engine considers each subdomain as a new site. Therefore, the subdomain site will need to be optimized as the main site by creating back-links, battling the sandbox, and all other.