Getting more backlinks, will this help in getting higher SERP or not? Or are the backlinks only for getting higher PageRank?
Back links essentially enhances your Page Rank and does not have a direct bearing on the search engine rankings (though it cannot be completely ruled out). Thematic back links do help in generating qualified visitors and if that is the motive then you should pursue it.
SERP rely on your use of keywords and how they are arranged and positioned in your content. Only google knows how to tell in detail on that. Your PR however is mainly on the amount of backlinks pointing to your site though some experts actually argue that they do affect on the SERP but don't know to certain extent.
that's bull. it is all about trust and links. when your site is trusted enough placing link will get it to the #1.
If you get backlinks, then your SERPs and PR, both will increase. How much they will increase? depends on quality of backlinks.
For an answer to your question, read my guide to link building. Also, consider this. SEO can be divided into two relatively distinct parts - On page and Off page factors. I like to see SEO as a racing car, with off page factors acting as the engine and on page SEO factors taking charge of the steering. Backlinks give you power (PageRank) while your trajectory towards your chosen niche is primarily determined by the content of your web site and the many things that Google and the other major search engines pay attention to within it.
Don't worry about whether it will effect page rank or serps. Too many things relate to it and you definitely don't have much resources and time to check all of that. What you should be concern and understand is the concept of online marketing is about to promote your link to as many places as possible and to make conversion if you sell something when it matters. Link is an asset whether it is dofollow or nofollow. They both has their own benefit and usefulness.
quality link is most important, i have a site which only have 500 links to it, but these links are there for about 2 years, another site 5k links to it, but the site with 500 links rank much better then the one with 5k links, quality link is most important, I think quality link will include age of links, relevancy, good pr links