you can get the backlinks from various sources such as: 1) blog postings 2) forums postings and signatures 3) directory submissions 4) article writing 5) press releases 6) classifieds 7) social bookmarking 8) review websites 9) postings tags 10) postings in guest books Google's way of calculating backlinks is unknow but somethings are clear like if a noindex or nofollow is used then it will not count it in the list of backlinks, if it is a spam or irrelevant then also it will not be counted
i dnt knw hw does google calculate back links but directory submission and asking links by sending mails basically to them who has there domain name same as your keywords might help you.
Normally we get back links from Article submission Directory submission Social bookmarking Forum posting Adds submission Press release But google will calculate the back links nobody knows..