Grimey said:
Is a catechumen some kind of pokemon? Clue us non-Orthodox in on what this means.
A catechumen is someone who receives instruction on the Church's teachings in preparation for joining the Church.
The instruction begins around the start of the Church calendar (September 1st) and culminates in joining the Church just before Pascha/Easter.
k2 can fill in more historical details, but I believe that the Didache was used as instruction for catechumens in the first couple of centuries in the Church.
As heresies multiplied the need for more formal instruction arose, so for about 1700 years now you've had more detailed instruction than what is provided in the Didache.