What OKRs mean
The method comes from Intel. Andy Grove, who later became Intel's CEO, described it in his book High Output Management through two questions: where do I want to go, and how will I pace myself to see if I am getting there? The first answer is the objective, the second gives the key results. John Doerr learned the method from Grove at Intel and took it to Google. According to Google's re:Work guide, he introduced it to Google's leadership in early 2000; Doerr's own company, What Matters, gives the year as 1999.
The objective is qualitative. It describes a change that matters, such as "the blog brings in customers, not just visits". Google asks for objectives that are ambitious and feel somewhat uncomfortable. Key results are quantitative. Each names a figure that can be checked at the end of the period, such as "from 40 to 70 demo requests a quarter that start on the blog". If nobody can say at the end whether it was achieved, it is not a key result.
Google's guide recommends a small number: three to five objectives for an organisation, with about three key results each. More objectives spread the team's effort thin. OKRs are public inside the company, so every team can see what the others are working on and align its own goals with the organisation's.
A KPI is an indicator that is watched continuously, such as a landing page's conversion rate or a campaign's ROI. It tells you how the business is doing. A key result sets how far that indicator should move in a given period. A KPI can measure a key result but does not replace it: "conversion rate" is a KPI, while "raise the demo conversion rate from 2% to 3% this quarter" is a key result.