I will update this with proper descriptions from the Guide Book but for now, here's my vague description
Difficulty - A measure of how difficult the river is to paddle, this is generally described as being the river grade (eg: the Boyne is Grade 2, the Annamoe is Grade 3, Clare Glens are Grade 4, etc). This can also be written with a + or - following the number to indicate that it's on the upper / lower end of that grade.
Grade 1 - Flat Calm Water (eg: Swimming Pool, Shallow Lake, etc)
Grade 2 - Flat but moving Water (eg: Liffey, Boyne, etc)
Grade 3 - Some Rapids, requires boat skills to navigate (eg: Annamoe)
Grade 4 - Continuous rapids, requires well developed skills (eg: Clare Glens, Dargle, etc)
Grade 5 - Very difficult, requires careful preparation and planning by highly skilled kayakers
Grade 6 - Maybe think twice about this
Consequences - A measure of the consequences that will be faced by a paddler who makes a mistake on this river. Gives a better understanding of a river, a Difficulty 2 river could easily have very high consequences if the paddler makes a mistake.
A - Little / no consequences
B -
C -
D -
E -
F - Extremely Dangerous
Together: Together these give an overview of a river. A river with a difficulty of 2+ and consequences of C would be written as 2+/C.