Link building is the active process of pursuing and earning backlinks through outreach, guest posting, digital PR and other proactive tactics. You go after the links. Link earning is the passive process: you create content so good that other sites link to it spontaneously, without you having to ask.
In practice, the best strategies combine both. Pure link earning is ideal, but it takes time. Few sites earn enough spontaneous links on content quality alone, especially early on. Active link building accelerates the process while authority is being built.
The healthy balance is investing in high-quality content (linkable assets that generate natural link earning) while doing editorial outreach (active link building) to strategic publications. Over time, as authority grows, the share of spontaneous links tends to increase.