--Bona al la encycla lorica.
Spellcaster: Generic Definition
A spellcaster in the widest sense of the term is anyone who casts spells, and thus refers to anyone in the maging community. Commoners - who tend not to know better - thus usually use "spellcaster" to refer to anyone casting their "magic", which for them is also vaguely defined. The way the vast majority of them see it, there's really no difference between someone who casts spells and someone who does anything more than cast spells. This does tend to result in a bit of confusion for anyone who does care, however, like the rest of the maging community.
The maging community consists of only 0.10% of the world population (100,000). However, only about half of them actually wield competent magic (instead of just the most basic potion-based spells). These competent spellcasters comprise a mere 0.05% of the world population (50,000). It is this startlingly low number compared to the general population that makes spellcasters a rather rare sight.
Spellcaster: Specific Definition
In particular, spellcaster refers to one of a trio of spell-related professions, along with spellcrafting and spellshaping. In this sense, "spellcaster" refers to one who only casts spells, and neither creates them nor invents them.
An analogy to this is the swordsman. Swordsmen use swords, but they don't make swords and they certainly didn't invent or improve on the sword design. In this analogy, the spellcasters are like the swordsman.
The threshold for becoming a spellcaster isn't that high. At its simplest, "just point the wand at the enemy and press the key" does nicely as an example of how simple spellcasting can be. Spellcasters tend to be lacking in understanding of magic. And as far as the guilds are concerned, it's best that their spellcasters aren't in the know. They don't want their high magic secrets to get leaked, after all.
Joining as a Spellcaster
The role of spellcaster is the least of the guild's three primary roles, and so the guild sets a low bar for becoming a spellcaster. People of any age can join, provided they can demonstrate that they are sound and competent of mind and body, as would be expected from someone entrusted with ensuring the safety of the guild. Should the applicant pass, he/she becomes accepted by the guild and becomes an initiate, a one-year-long boot camp of sorts that gets the initiate into physical shape, trains them in the ways of casting spells and indoctrinates them. Teaching them to cast the spells generally takes the least amount of time. Once they have completed their first year they become apprentice spellcasters.
Those spellcasters working under the guilds usually fall into one of two types.