A spell capable of being cast by all mages is generally thought of as 1st Tier Magic or 0 Tier Magic, while a spell that belongs to 2nd Tier was more potent, requiring a better mage and longer periods of study to learn it. 3rd Tier magic is generally the bar for most mages, and they would know only one or two spells of that level. Anything higher gets into the range of extremely powerful magic, exponentially stronger and just as hard to learn.
To cast spells of specific tiers it usually requires for the mage to have reached a certain level.
However, lower-level mages can cast higher Tier magic with enough practice and time.
One thing to should be noted is that Ceria received the knowledge of how to cast [Fireball], a Tier 3 spell, when she reached Level 18 in her [Elementalist] class. It is not known, however, if all magic related classes gain a spell of that tier upon reaching said level.
[Mages] can level by solely casting spells, but it is not very efficient. Unlinke studying spells, which allows a mage to gain a lot more experience, making it far more likely to level up using that method. 
According to Pisces, there are seven, or eight tiers. Apparently there is the speculation of the ninth tier , but no mage has ever cast or discovered a spell of such magic.
There are currently 6 known archmages, "the rarest of spellcasters". "One in the Human lands to the north. Two more in other continents. But at least three have homes in Wistram, the Isle of Mages.” However, none can be found in Izril.
“You fly through the air like the greatest of [Archmages] I am told.” This might mean that usually mages that can fly are rather restricted and only [Archmages] can fly as high and fast as a plane does.
“Long distance teleportations usually require a spell circle and time. If a person teleports instantly, it means that they haven't travelled very far, and are going to appear at a prearranged destination, making it rather easy to locate them.