Subdomains can be very useful for SEO, but only in certain situations.

Note that search engines to some extent treat a subdomain as a separate domain (website). Thus, the page rank, link popularity, authority, etc of your primary domain will not necessarily be transferred to the subdomain. This means that if you use subdomains, you need to have specific link building plans for that subdomain. If you want to use lots of subdomains, this means lots of extra work building links, and diluting your overall link building efforts.

Also, be sure any content in a subdomain is largely unique. Given search engines treat subdomains as separate websites, duplicate content filtering can kick in, hurting both the subdomain and any other domain that has that content (including your main site).

On the other hand, using a subdomain can have good search engine value, mainly from having keywords in the subdomain (e.g., keywords.mydomain.com).

Thus, only use subdomains if you are creating a unique section of your site that has specific value, and you are prepared to put some effort into link building for that subdomain, as well as your other domains.

One site that does very well with subdomains is about.com. They have hundreds, and use keywords in the subdomain very effectively. However, they have lots of link authority to pass around, and tons of unique content.

If your entire website is about the same topic, I see no point in using subdomains. However, question of my own ...

Do you spread more link love between pages of a single domain or between subdomains of a site? It's always been my experience that pagerank flows easier among pages of a single site, but an coming link from a unique domain counts more. So if you have a common navigation among various subdomains of a site, does that count towards the number of incoming unique domain links?

I don't think that links between subdomains (including www. to/from other subdomains) counts nearly as much as links from an external domain, and probably do not count as true "external" links. It's a little like links from sites on the same IP address -- it is assumed you control them all, so the "weight" of the link is more like an internal link. I don't know for sure if links between subdomains count more or less than within a domain, but I suspect it is less, since I've done this type of linking before, and the subdomain pages did not rank as well as internal pages with similar level of links.

So, yes it is good to interlink websites that are subdomains, but you need true external linking to each subdomain to really build link popularity.

My idea was to use the sub domains in order to insert in the domains my key words
I mean my website url regretfully has not the keywords that I like ... so maybe someting like

keyword.domain.net

this can help?

I have not thought to insert link from sub domain to my main domain in order to increase ingoing number of links .... I think will not work at all, will be considered internal link.

Using keywords in the subdomain name will definitely help positions, but as noted above, unless you do external link building on that subdomain it will still not rank well. If there are just one or two keywords you think are critical, it may work to define subdomains for those and do link building. However, those subdomains must serve unique content relevant to those keywords to rank well, and you need to keep them separate from your www.mydomain.net website.

Ultimately, the bulk of your rankings will come from focusing on good content and link building. Over time, having inbound links that contain those keywords in the link anchor text, and having those keywords in your page title, metas, content, etc, will do much more for your rankings. You may be better off going that direction than setting up subdomains that each need separate content and promotion to work well.

I was never one for using keywords in my URL. I would rather have a catchy domain name that is brandable and conducive to word of mouth as opposed to something that strings together a bunch of search keywords.

the page names can cover a lot of that.... as well as strategic subdirectories

that used to be the case and the branding thing is still true but look at Google search results... the domain name as a keyword gets a lot of play these days

Ah, this is always a hot topic at the conferences.

Think of subdomains as a way to organize your site's structure, rather than as a means to to rank better. Great examples of sites that properly and effectively use subdomains are craigslist (organized by region), wikipedia (language), and the NYT (section).

For what it's worth, Matt Cutts of Google "says" that keywords in the URL's may help rankings.