Most wizards pre-select their spells; if a wizard can memorize two 2nd-level spells, he picks two specific 2nd-level spells from his spell book (say, invisibility and wizard lock) and stores them in his mind. This is referred to as a fixed magick; once chosen, these spells cannot be changed until the wizard casts them and then studies his spell books again.
The other option is Free Magick. A free magick is one which the wizard does not place a specific spell into. By keeping the magick open, the wizard can use it to cast any spell of the appropriate level that he has in his spell books. For example, a 3rd-level specialist wizard might spend his spell slots to acquire one fixed 2nd-level magick within his specialization (invisibility, for instance) and one free 2nd-level magick. With the free magick, the wizard can cast any 2nd-level spell in his book when he needs it. If the wizard found himself in a situation where he needed to probe someone’s thoughts, he could use ESP; if he needed to aid a friend involved in a fight, he could use strength; or, if he needed to distract some pursuers, he could use improved phantasmal force. As long as the spell is known to the wizard and recorded in his spell books, it’s available through a free magick.
Obviously, this is a huge advantage for a wizard. Instead of guessing about which of his spells may be useful in a particular adventure, the wizard can have all of his spells of that level accessible. However, free magicks are less efficient than fixed magicks and cost double the appropriate spell slots to fill. Once the free magick has been used to cast a spell, it is wiped from the wizard’s memory, just like a fixed magick.
The same ability is available to priests in the form of Fixed and Free Theurgies. Spells may be chosen as pre-memorized fixed theurgies (the priestly equivalent of magicks), or the priest can leave a theurgy open to use any spell he wishes to by selecting a free theurgy (see above description for wizards).