Registered voters are those who in response to a standard poll question say they are "registered to vote in their precinct or election district." This is the group whose data Gallup reports most often because they represent an estimate of Americans who in theory are eligible to vote and could vote if they want to.
Of course, Gallup knows that in the final analysis, not all of these registered voters will actually vote. So Gallup has over the years created systems to isolate likely voters -- that group of individuals who the company can estimate are most likely to actually vote.
There are many ways to estimate likely voters. Other pollsters and polling groups have different approaches. Gallup has spent decades developing its system, which the company has found in election after election helps improve accuracy in terms of how the final poll before an election compares with the actual vote percentages on Election Day itself.
Learn more about Gallup's likely voter models.