I want , because to have site more organised, to have subdomains...
For site categories per example (new cars, used cars and other stuff) i want to have newcars.mydomain.com, used.mydomain.com and cettera...and then from menu linking to that subdomains.
Instead in menu link (www.mydomain.com/newcars/) I will use (www.newcars.mydomain.com).

How is that working with SEO?
How Google looks at that?

subdomains confuse users,
subdomains are treated as independent sites

http://www.webmaster-talk.com/google...com-forum.html

IMO it is more useful for SEO.
Google "Yahoo" and you will get bunch of yahoo links.Which is because of subdomains.

I think you see all those subdomain yahoo sites when you search for yahoo because yahoo has been around for quite awhile and probably has as many if not more links pointing to the site and it's subdomains than any other site online.

Also who else would you expect to show up when searching for yahoo. You'd see those sites listed if they used subfolders instead of subdomains.

I'm with Chris on this. Search engines will treat subdomains as independent sites so you'll be diluting your seo efforts. There are reasons to use subdomains on a site, but typically you'll need a rather large site before it makes sense.

Are Add-on domains also considered sub domains? They have a seperate url... Will this also be confusing or diluting to the search engines? How effective will SEO and web site promotion be on an add-on domain?

The reason I ask is because I currently have 2 add-on domains on my hosting account. I have it this way because I want them to appear as seperate websites, while obviously saving money on hosting.

if they have a seperate URL, something.tld not something.mainsite.tld they are NOT subdomains.

i have had very good results using multiple subdomains to target a different keyword on each one however i have had problems when put my links directories on sub domains. If you are planning to use a reciprocal linking scheme be very careful when using subdomians.