If this question has already been asked I couldn't find it. On the DB it seems to describe super effectiveness as double damage and ineffectiveness as half damage. Is that a literal representation of what the damage bonus is?
Super effective moves are 2x damage.
'Reliable source' as you requested...
Super Effective means double damage, or x2. You do twice as much damage.
Super Effectiveness:252 SpA Life Orb Greninja Hidden Power Water vs. 252 HP / 4 SpD Heatran: 205-244 (53.1 - 63.2%) -- guaranteed 2HKO
Normal Effectivness:252 SpA Life Orb Greninja Hidden Power Dark vs. 252 HP / 4 SpD Heatran: 103-122 (26.6 - 31.6%) -- guaranteed 4HKO
26.6 - 31.6% is for normal and 53.1 - 63.2% is for super effective. See how super effective is 2x as much damage as normal? It does twice as much. I am doing not very effective for your convenience,
Not Very Effective:252 SpA Life Orb Protean Greninja Hidden Power Psychic vs. 252 HP / 4 SpD Heatran: 51-61 (13.2 - 15.8%) -- possible 7HKO
2x less chance to KO than Normal Effectivness, except for that 7HKO, but see? It does half damage.
Hope I helped!
Yes, it is double damage.
Yeah. Simply it just doubles the damage that you do.