The process of establishing relevant, inbound links to your website which help your website achieve higher ranking with the major search engines and drive targeted traffic to your site refers to link building.
SEO (Search Engine Optimization) falls into two main categories:
On-page SEO: On-page SEO is comprised of the factors you change on your actual Web site itself.
Off-page SEO: is essentially comprised of everything you do elsewhere to promote your blog with search engines in mind. Link-building falls under off-page SEO and is one of SEO’s most important aspects. Consequently, it’s also one of the most controversial, convoluted, and complex. In its simplest form, link-building can be defined in the following way:
A Simple Definition of Link-Building: The process of generating a link to your Web site on another Web site.
Below are some important reasons why you need to hire SEO link building services:
• To get high returns on marketing investments: Many successful websites depend on link building services for increasing their sales revenues, and it would make a lot of business sense
• Professional resources: If you were to do it in-house, primarily you will need to have proper knowledge and understanding on intricacies of search engine optimization. Moreover, it is a serious time consuming job which actually begins with choosing a domain name for your website, and then designing your website by using the appropriate contents, which include the keywords in acceptable densities. The contents should also be compelling enough to convert your web traffic into customers. It is only then the real link building begins. However, it would be a wise idea to delegate this important process to the professionals, so that you could be focused on the core areas of your business administration.
• Staying ahead in the competition: It means that you could reach out to your prospect customers before your competition does. Companies offering SEO link building services will be constantly analyzing the keyword trends and demands. Any good company worth its salt will incorporate the latest market changes in their promotional strategies. Therefore, it becomes all the more reason for you to hire their services, in order to stay ahead of you competitors.
• Good SEO link building help you in getting better organic rankings: Unlike the paying options, the organic search engine rankings will stay consistently on the top for longer durations. These rankings are very important for any website, since most of the targeted traffic comes from search engines. The SEO link building services will help you get better page rankings, and more importantly they help you stay on the top for a long time.
Steps for link building:
 Easily Manage Multiple Link Building Campaigns and Data
 Quickly Discover Relevant, Valuable, Quality Link Prospect
 Find Relevant Competitor Backlinks, by Topic or Link Type.
 Content Marketing Research for Brainstorming and Promotion
 Monitor and Visualize Competitor Backlink Scores Over Time

Furthermore, to conclude Link-building is one of the most important factors of SEO. This acts as guide and explains exactly what link-building is, how to approach it, and what you should avoid.

Link Building is a primary aspect of SEO of any website and it is always seen that a well-optimized website has the effectual links associated with them. The main reason is that links make connection between the pages on websites and make it a point for the web spiders for gauging the website, through different keywords or key phrases. The number of links along with their quality and effectiveness is the major aspect for the website to look forward for their link building campaigns.

You can also called Link building off-side seo.Link building includes.
Directory Submission
Article Submission
Social Bookmarking
Blog Commenting
Form Commenting etc.