Straight: Marcy is a magic-user whose spells can only be used so many times per day, and have specific limits on what can't be done with them.
Exaggerated: Marcy's spells have so many rules governing their use that it's often more convenient for her just to use mundane solutions to her problems.
Downplayed: Marcy's spells have guidelines they must follow, rather than set rules.
The gods who made the world don't want mages to break it too easily, so they put limitations to hold magic back from being too easy to use.
It's just sufficiently advanced science, and so follows laws similar to the rest of physics. See deconstructed.
Magic is chaotic in nature—it can do anything, and sometimes does, except that it's not often the effect the mage desired, and may suddenly stop working for no discernable reason. Anyone who tries to determine the laws of magic goes mad.
Double Subverted: Boris is a member of the Arctic Brotherhood, who use a slightly different source of magic than Marcy's guild, and thus have different rules. There are things Marcy can do that Boris can't, and by pooling their knowledge, the two can learn which rules are vital and which are just tradition.
Parodied: The Laws of Magic are created by an actual Magical Congress, which results in them being as complicated as real-life legal codes. Marcy is forced to buy new volumes of regulations every year so that she can learn the rules and continue to use magic.
Zig Zagged: Marcy is introduced on Generic Fantasy Show by a writer who doesn't bother to think through the implications of her magic. When other writers use her in the first season, her powers work as the plot demands without reference to how previous episodes handled it. Eventually, one of the writers comes up with consistent rules for Marcy's magic, which some of the writers remember to use, others don't, and a few handwave special exceptions for. Worse, the episodes are not aired in the order they were produced, so Marcy may not have a spell this week that she had last week. Fans trying to write Marcy up for role-playing games have fits.
Averted: No one ever explains how magic works, or what its limitations are. It just is, and that's all anyone cares about.
Enforced: The writers of Generic Fantasy Game put strict limits on how magic works so that mages are balanced with other Character Classes.
Lampshaded: "Can you get us out of this, Marcy, or is this another of your 'Magic has limitations' moments?"
Invoked: The gods tame magic and force it to work properly.
Exploited: Maslak the Mage-Killer learns the rules of magic, so that he can use them to prevent wizards from using their spells against him.
Defied: "I can do anything with magic, as long as I have the imagination!"
Discussed: "Let's see, it's a Monday, so I can't use Divinations, but tonight is a full moon, which strengthens Silver Magic, and we're standing on sand, so the spells available to me are...."
Conversed: "Don't play a mage in this game unless you're prepared to memorize a lot of rules about the magic system. Beginners should probably stick to fighter types."
Despite having consistent rules magic is seen as too 'mystic' to be 'real science' and mages are looked down on by engineers and scientists etc. The mages, for their part, see magic as 'more art than science'.
The Mana that powers the spells comes from Physical Gods and the rules are consistent because they help teaching new mages how to use magic. They could change the rules on a whim and without warning.