What is the difference between a newspaper's news desk and its editorial line?
The news desk reports what happened. The editorial line is the publication's institutional opinion. In principle they are separate operations with a wall between them; in practice the relationship is more complicated, and understanding it helps you read any outlet.
The news operation is staffed by reporters and news editors whose professional norms require accuracy, attribution and — at least nominally — keeping their own views out. They are assessed on whether the facts hold up.
The editorial or comment operation is separate, usually with its own editor. The leader column (or editorial) expresses the publication's view, written anonymously and institutionally. Signed columns express individual writers' views and may contradict the leader.
Where the separation is real. Reporters generally do not write leaders, comment writers do not assign news coverage, and reputable outlets treat interference as a serious breach. Journalists frequently and publicly disagree with their own paper's editorial line.
Where it is more porous than the formal structure suggests:
Story selection. The editorial stance influences what is considered newsworthy, and this is the most consequential channel. Choosing to run a story prominently, or not at all, requires no factual distortion.
Framing and emphasis in headlines, which are written by subeditors who work within the publication's culture.
Hiring. Over time, staff are recruited who fit the outlet's outlook, so the line is reproduced without anyone being instructed.
Proprietorial influence, which varies from none to considerable and is rarely visible.
Broadcast is different. In the UK, broadcasters are bound by Ofcom's due impartiality rules and the BBC by its charter, so they may not take editorial positions on controversial matters in news output. Newspapers are under no such obligation and never have been.
Practical reading advice: check which section a piece sits in, and notice what an outlet chooses not to cover.