Recently a user told me to avoid subdomains when i can. I remember reading google consider subdomains as a unique site (is this true?). What else happens when i use a subdomain and when should i use or should not use a subdomain?
I heard cookies are not shared between subdomains? i know 2 images can be DL simultaneously from a site. Would i be able to DL 4 if i use sub1.mysite.com and sub2.mysite.com?
What else should i know?
You can share cookies between subdomains, provided you set the right parameters in the cookie. By default, they won't be shared, though.
Yes, you can get more simultaneous downloads if you store images in different subdomains. However, the other side of the scale is that the user spends more time resolving DNSs, so it's not practical to have, say, 25 subdomains to get 50 simultaneous downloads.
Another thing that happens with subdomains is that AJAX requests won't work without some effort (you CAN make them work using document.domain tricks, but it's far from straightforward).
Can't help with the SEO part, however, although some people discourage having both yoursite.com and www.yoursite.com working and returning the same content, because it "dilutes your pagerank". Not sure how true that is.
You complicate quite a few things. Collecting stats, controlling spiders, html5 storage, XSS, inter-frame communication, virtual-host setup, third-party ad serving, interaction with remote APIs like google maps.
That's not to say these things can't be solved, just that the rise in complexity adds more work and may not provide suitable benefits to compensate.
I should add that I went down this path once myself for a classifieds site, adding domains like porshe.site.com, ferrari.site.com hoping to boost rank for those keywords. In the end I did not see noticeable improvement and even worse google was walking the entire site via each subdomain, meaning that a search for ferraris might return porsche.site.com/ferraris instead of ferrari.site.com/ferraris. In short google considered each site to be duplicates but it still crawled each site every time it visited.
Again, workarounds existed but I chose simplicity and I don't regret it.
If you use sub domains to store your web sites images, javascript, stylesheets, etc. then your pages may load quicker. Browsers limit the number of simultaneous connections to each domain name. The more sub domains you use, the more connection can be made at the same time to collect the web pages content.
Recently a user told me to avoid subdomains when i can. I remember reading google consider subdomains as a unique site (is this true?). What else happens when i use a subdomain and when should i use or should not use a subdomain?
The last thing I heard about Google optimization, is that domains count for more pagerank than subdomains. I also believe that page rank calculations are per page, not per site (according to algorithm etc.). Though the only person who can really tell you is a Google employee.
I heard cookies are not shared between subdomains?
You should be able to use a cookie for all subdomains. www.mysite.com sub1.mysite.com sub2.mysite.com can all share the same cookies, but a cookie specified for mysite.com cannot be shared with them.
i know 2 images can be DL simultaneously from a site. Would i be able to DL 4 if i use sub1.mysite.com and sub2.mysite.com?
I'm not sure what you mean by DL simultaneously. Often times, a browser with a single thread will download images one at a time, even from different domains. Browsers with multiple thread configurations can download multiple items from different domains at the same time.