Link building is acquiring hyperlinks from other websites to yours. Search engines interpret backlinks as endorsements. A high-authority site linking to you = vote of confidence = improved ranking. Outreach is the strategy: find websites to pitch, create value propositions, secure links. Tactics: guest posting (write article for other site, link back to yours), resource pages (get listed in "best tools" roundup), broken link replacement (find broken link on authoritative site, pitch your content as replacement), relationship building (establish rapport, earn link naturally).