Assume your website = TREE, Back Links = Root of Tree.
When we say a Tree is big and can stand in front of any environmental calamity it means the ROOT of the Tree is deep bellow the soil. Deeper and more spread the root is means the tree is much stronger.
Website back link aspect works in the same way. More proper back link (will explain bellow what proper back link means) means more strong the website is and hence Google and other Search engine loves the site and your site become much more popular.
Proper Back link:
Back-link means - your website URL (with targeted keyword) how many other websites place in there website content. Here some factors need to keep in mind
1. The website where we will place our URL must be in relevant business vertical. Assume you have mobile website so you like to place your website link in other mobile related website. Not in sex, or travel or other vertices website. For this website Card related website will be preferred to get much benefit.
2. The PR or page rank of the page from where we will get the back link. Higher the PR value more benefit for your website. PR starts from 0 to 10. So assume if you get the back link from PR 6+ that means your website is getting proper benefit. Search Engine Marketing effects by this link building aspect.
3. The popularity of the site where we are placing the content and how we are placing our content. It should be natural. Like Commenting in others post using URL is the best way to increase the back link. But those comment need to be proper and relevant not spamming.
4. Back link - creation another way is to create proper Blog and maintain that and place there proper content and populate the blog using RSS etc is another smart way to increase the back link.
5. Also the server status of the website where we like to place the link is also matters. There are 3 kind a IP Class A, B C and as per those classification the quality of the link changes.
There are other factors as well. In Nutshell one need to place there website link in other relevant website as much as one can and with proper logic and reason why.
Note: One crucial thing need to keep in mind. We can"t increase the Backline in vast way. Assume you though currently your back link 10 you want to make it 1000. So assume you place 1000 links in other site in 7 days time. It will place a negative impact for search engine and Google will penalize for these. So slow but steady way. Same way like having daily food in limited way, in particular time.
We tried to explain the Backlink aspect the simpler way i can. If you have any questions then please help us to know.