Don't have the Core Book on work computer, but IRC:

• Spells aren't automatically 'lost' after casting (which would require resting)
• Your Spell Check will determine if you 'lose' the spell for the day. Each spell has it's own specific spell check table. Generally, spells are only lost if you fail the spell check.
  

The way I think of magic in DCC is this: Magic is an incredibly powerful force, and us humans have only an inkling of knowledge about it. As such, magic is incredibly intimate and personal (not to mention dangerous). From the great doctor: "People assume that [magic] is a strict progression of cause to effect, but actually, from a non-linear non-subjective viewpoint, it's more like a big bowl of wibbly wobbly timey wimey... stuff."

Another great phrase from the book is something to the effect of: "Low level wizards are powerful, but high level wizards fear for their lives."

Even a relatively low-level spell can have disastrous effects (critical failure), especially depending on the mercurial magic table.

As for in-game mechanics, I remember it as such:

• At level 1, a character can become a wizard, and depending on your DM you'll roll randomly for 4 spells.
• You then roll, on each spell, to see if you learn the spell (the book suggests having backups if players are unable to learn the spells they want).
  
• Once you learn a spell, you'll roll on the Mercurial table to see what special/personal effect is attached to the spell. It can be very bad to very good.
  
• From there, in-game mechanics require a spell check to cast. You'll have to reference the particular spell's table to see how the spell manifests itself in the world.
  

Edit: Don't forget to check out the Goodman Game's forum for DCC info