Definition:
Practice of acquiring links pointing to your website from an external site.
Links pointing to your site from another site are sometimes referred to as "inbound links", "external links", or "backlinks" (one word). In contrast, links that point to webpages on the same site are called "internal links".
Link building and online marketing are almost synonymous. Often, the point of online marketing is to get a link from someone else's website.
Examples: Your link building strategy should consider the relevancy and popularity of the website where the inbound link is coming from.