What's the difference between Unionists and Loyalists in Northern Ireland?

Question

As I understand it, both groups advocate for the United Kingdom's retention of the six Ulster counties. What I'm struggling to determine, however, is the difference between these two groups/ideologies. In some contexts, these terms seem to be used interchangeably:

Yet despite their protestations, there is little those most loyal to the British Crown can do. There has not been a successful grassroots protest by the Protestant-unionist-loyalist population against British government policy since they toppled the Sunningdale devolved power-sharing arrangement in 1974.

While in others, loyalists seem to be characterised as distinct groups, the loyalists referred to as militants, while the unionists seem to refer to the formal political wing:

The physical strain and the disruption to his personal life can only have been made bearable by the knowledge that peace was within grasp: the IRA and the loyalists were on ceasefire, the unionists had a leader who was prepared to make a deal despite strong reservations, the blueprint for a settlement had already been laid out by John Hume and there were political leaders in Dublin, London and the White House giving the process their full backing.

Is this accurate, and what are the differences between these two groups?

Edit: Another article entitled "Brexit: Unionists condemn threat from loyalist paramilitaries to implement violence" would seem to reinforce this distinction:

Unionists have said there is "no place for violence in our society" after loyalist paramilitaries threatened protests if Northern Ireland's status in the UK is "diluted" after Brexit.

Answer 1

Loyalism is a major current within unionism. Even more subtle distinctions can be made within the two between "new" and traditional forms of each. From the book Unionists, Loyalists, and Conflict Transformation in Northern Ireland By Lee A. Smithey:

Unionism has often been characterized along two general lines: old, traditional, or conventional unionism and new, liberal, or civic unionism [...] Conventional unionism focuses on the constitutional relationship linking Northern Ireland and Great Britain alongside a deep appreciation for British identity and the historical empire's military and industrial strength. [...] "New unionism" refers to attempts to shake conventional unionism from its fixation on Northern Ireland's constitutional status and provide rationales that allow for greater political maneuverability (p. 57).

[...]

Loyalism has often been defined as physical force unionism that was primarily situated in working-class areas. [...] More inclusive definitions of Ulster loyalism have included loyalist paramilitaries, the loyal orders, and evangelical Protestants [...] New loyalism refers to the emergence of politicians and political organizations connected to loyalist paramilitaries, in particular the Progressive Unionist Party (PUP), which has drawn leadership from the Ulster Volunteer Force, notably David Ervine and Billy Hutchinson (p. 58).