I have heard many people ask over the years, "Why is it a wizard may have prepared a fireball spell everyday for years and not recall it on the fly?" Sometimes, I feel that people lose the sense for fluff explanations for how Pathfinder works mechanically. Now these may not be exactly how the creators intended for things to be done, but I'm gonna run with it anyways. Also, many should note that these explanations were created with Golarion in mind, since it is the only setting I play anymore.
Concerning Spellcasters
*General Spellcasting for Humanoids - Unless they have a tie to some sort of magical power, humanoids (mainly humans, others may be an exception) are unable to cast arcane spells innately, although there are some methods to get around this (see below).
*Sorcerers - Having ties to magical creatures through a bloodline is one way humanoids can break this rule. The blood that courses through their veins have magical properties that allow them to create magical effects through seer force of will. For the most part, arcane spells cast by a sorcerer are the original version of the spell (see the Wizard explanation below for further info).
*Wizards - Wizards are another exception to the rule. Yet, instead of being able to cast spells due to a bloodline, they instead study the finer workings of magic, breaking down the essential parts to recreate certain effects. Each spell they learn they scribe into their spellbook, yet, what they scribe in their book isn't actually the spell. Instead, it is a minor ritual that allows the wizard to create the spell's effect and store the energy in their body to be utilized later. As the wizard progresses, he is able to hold more magical energy in his body to release as spells. This is the reason a wizard, no matter how many years he's been preparing it, can't cast a spell out of no where (at least not without specialized training such as feats and such). If he did the minor ritual for fireball twice, he contains within his body the energy to create the fireball spell's effects twice. He can't cast it a third time due to that he didn't summon up the energy necessary via the ritual to hold a third fireball. In addition, the effect that the wizard releases isn't actually the spell in question. Instead, when he prepared the minor ritual, he learned that if regent A mixes with regent B, a certain effect will take place, and if that conglomeration mixes with regent C, an effect similar to fireball will be created for use. For all intents and purposes, the wizard just uses the fireball spell.
*Alchemists - Alchemists go about creating spell effects in a similar way to a wizard. They have spell effects listed in the form of a recipe. They know that if plant A is added to plant B to create mixture A, and if solution A is added to solution B to create mixture B, and the two mixtures are put together, they create an extract whose effects are similar to whatever spell (let's say cat's grace). The extract is not exactly cat's grace, but for all intents and purposes, the alchemist just uses the cat's grace spell.
Concerning Hit Points & Attacks
*Regular Attacks - When a character attacks a creature or another humanoid, they have the chance of doing damage. Let's use modern day martial arts for example. In the ring, you will notice that not every attack hits the target. When an attacker does hit their target, sometimes it's not exactly the target they were aiming for. Nonetheless, the attacker deals damage. Now, the attack, although it dealt damage, may not have hit flush. This may result in the attacker rolling bad on the damage roll. When the attacker does hit flush, this would be akin to the attacker rolling high damage or max damage. If we were to apply this to weaponry such as the type prevalent in Pathfinder, the attacker could either miss entirely or connect but not do any damage; the fighter's sword connected but was minorly deflected off the opponents armor. A minor hit would be a fighter hitting his target and although the fighter connected with the attack, the opponent moved slightly and the attack didn't do very much damage (think of a martial artist throwing a jab and connecting, but his opponent moved back just milliseconds before the attack landed and it only grazed his nose); the target was hit but the attacker rolled poorly for damage. A solid hit would be the fighter hitting his target and the opponent was in a prime position to feel the entire force behind the attack.
*Critical Hits - A critical hit would be akin to a fighter shoving his sword through a body part or torso of his opponent (this is dependent on the weapon). In a martial arts match, this would be akin to a blow that either rocked the target (making him wobble a bit) or knocking him out cold (if the attack knocked the opponent unconscious).
I would like to hear the Pathfinder community's thoughts on fluff explanations to mechanical rules. Feel free to change mine or add to them to further explain, or go into detail about other aspects of the Pathfinder role playing game. I'm very interested to see everyone's thoughts.
You also need the power to cast the spell and I am thinking that memorizing the spell includes setting it up to be cast later. You can always make up flavor to make the mechanics make sense.
I can always give flavor for a rule if needed but I almost never describe it while playing.