Could someone please explain how Attack and Defence affect Monster Stats and how Spellpower affects Spells?
Every creature has a base Attack / Defense score.
When combat starts, your creatures get a bonus to Attack / Defense equal to your Hero's Attack / Defense scores.
If a creature's combined attack is greater than their target's defense, they take an additional 5% damage per point of difference, up to a cap of +300% damage (when attack is 60 greater than defense)
Conversely, if a creature's combined attack is less than their target's defense, they deal 2.5% less damage, down to 70% (when defense is 12 greater than attack).
Spellpower's exact effect varies depending on the spell. For instance, Magic Arrow does (10 * spell power) + 10 damage.
Haste has a constant effect and lasts Spell Power turns in duration.
For the spells, have a look at these pages, which describe each spell and how the spell is affected by spell power (in terms of turn duration or effect).