Its kind of like high school. More friends you have, more popular you are going to be. More popular those friends are, even better. Same thing with links. No link is a bad link, but relevant links from high PR sites are music to my ears.
URL is simply means all the links coming to your site from another sites. This is very important parameter for SEO because as much you can get the back links your SERP will be good.
backlinks are the links, u can do it as by posting your links in another site. by this u r site comes in search engines.