This is all from memory, since I don't have the book at this moment but here's a very broad overview. It basically works like this...each mage has a certain amount of components per day that he uses to construct a spell. Things such as range, area, damage, targets, etc. are bought with the components and the difficulty of casting the spell(a spellcraft check) is derived through the total cost of the components and other modifiers. With each spell you also risk suffering a certain amount of fatigue(especially if it's a more powerful spell) from casting which can make each successive casting harder.
Rotes are prepared spells that are pased down from mage to mage and thus easier to cast than an on-the-fly spell. A rote is an already constructed spell that, when cast, gives the mage a +5 to his spellcraft check, if the rote also corresponds to your particular path(necromancer, enchanter, warlock,etc.) then youy recieve an additional casting bonus of +5. You can freely use metamagic feats(quicken,silent,etc.) on rotes, but changing anything else about the spell(increasing damage, number of targets, energy type, etc.) causes you to loose the rote bonuses and it is cast as a regular "constructed" spell. So Rotes can be used both as traditional "spells" or as templates, for quickly tweaking instead of building a spell from scratch, for quicker play.