MAGIC SKILL - Mages study the arcane lore and language of Magic. Magic skill determines level and power of spells they can cast.

CASTING SPELLS - To cast a spell a Mage makes a Magic skill roll and expends 1 Power Point per Spell/Intensity Level:

Intensifying  Spell-effects can be increased by expending more than the minimum 1PP per spell-level, up to the caster's Max Spell Level.
Double Effect  Critical success allows the caster to double one spell effect, such as duration, range, area, etc., or to maximize the damage. 
Double Cost  If a casting roll fails, the spell still works but at twice the intended power cost (this may result in overcasting).
Overcasting  A caster having insufficient Power for their spell takes damage equal to the shortfall (to the head/skull), and the spell fails.
Cantrips  These are very simple spells of 0th-level, hence costing 0 Power to cast. They cannot be intensified, produce no critical, special or fumble effects, and take 1PP if the casting roll fails. Just 1 Memory memorizes all cantrips.
Initial Spells  During their apprenticeship, a Mage will have learned one 1st-level spell per point of INT above 10 (Detect Magic and Suggestion must be learned first) and will have up to one cantrip per point of INT.

LEARNING SPELLS  Mages can learn more spells by studying the relevant spell book, or 'grimoire'. Each day of study gives a chance equal to: Magic Skill - (10 x Spell-Level) %. This can be rolled for each day separately or accumulated for up to a month (30 days) and rolled once at the end. Researching spells, even new ones, is possible but takes longer and is much harder (see the main BRP rulebook).
Reference Books:  The standard fee for reference to spell books is 1 Gold Piece/day or 25 Gold Pieces/month, per spell-level.
Specialist Subjects: Specialist Mages (Illusionists, Necromancers, etc) learn spells of their chosen School at +10% but those of other Schools at -5%. They can intensify spells of their chosen School to 1 level greater than normal. (Specialism requires 14 INT, preferably 15+).

MEMORIZING SPELLS  To memorize a spell the Mage must study the relevant spell book (or grimoire) for a number of hours (i.e. spell-level + 1). This can automatically displace a spell currently in mind, if desired, or occupy any available vacant memory.

INCREASING POWER  Overcoming another creature's Power, usually by casting a spell on it, can earn a Power Gain roll. To increase Power by 1, roll over current Power x5%. A Mage can transfer Power gained into their Magic Staff or Familiar.

Magic Staff  Mages create a magic staff (or other form of fetish) as the master piece which completes their apprenticeship. This requires the sacrifice of much Power (hence the low initial Power of Mages). The lengthy creation process attunes the staff to the Mage, so that only they can use it (and they can only have one such attuned fetish item at a time). The Mage can transfer Power into their staff (but not back again). The staff acts as a reservoir of Power Points, which the Mage can use for casting spells when holding the staff. This Power regenerates itself independently ( up to the limit of the Mage's own personal Power, nightly). The staff can also store spells (i.e. as a grimoire), up to the Mage's INT. (Note that these spells will take 1 round per spell-level to cast, as from any grimoire). (A Staff costs 1cc at character creation.)

Familiars Mages often magically bind an animal of some sort to act as their familiar. The binding ritual requires the Mage to sacrifice personal Power equal to the Power of the creature. This ceremony creates a unique bond between the Mage and the animal, allowing a special empathic link between the two. When in close proximity, the Mage can use the familiar's Power Points and access spells it has memorized as if they were in his/her own mind (up to the Mage's INT/2). The familiar can use it's master's intellect (Int, Wis/Pow) and Defence when required to resist attacks. The two may communicate telepathically and the mage can give commands, use the senses and even speak through their bonded creature - but the latter uses cost 1 PP each. The Mage can also transfer Power to the familiar, though the familiar's Power can never exceed that of the Mage. (A Familiar costs 1cc, plus relevant Power loss, at character-creation).  

Armour Causes Fumbles  Mages avoid armour and large weapons/shields, as these items subtly interfere with spell-casting and make fumbles more likely. Burdensome armour or large weapons/shields increase fumble chance by +10/20/30% for Light/Medium/Heavy armour and +2xWeight% for weapons/shields over 3.5lbs.

Spell Books Each spell book contains details of just one spell. They are large tomes containing psycho-phonetic formulae for the spell, including detailed instructions as to the thoughts and gestures required, necessary material components and also copious notes and case-histories from which the spell's capabilities may be determined. In Pandaria, spell-books traditionally belong to the Guild of Arcane Lore. The Guild of Arcane Lore maintains Libraries in most major towns and cities, where Mages may reference the spell book of virtually any spell in existence (for a modest fee). As spell books are highly expensive most Mages completely rely upon this service - only very few (very rich) Mages can even attempt to maintain private libraries. Therefore Mages try to keep on good terms with the Guild by avoiding overly destructive or otherwise anti-social uses of magical power - in civilized areas, anyway...