Level

A level is an official kind of designation that Class cards can have, which determines how powerful they have become, representing the controlling player becoming more experienced with that class. The level can be increased by the "level abilities" of the class. There is no official marker or counter for this designation, although counters or notes could be used to track it informally.
Level is also an informal name for the number of level counters on permanents with Level up, known as leveler cards. These two types of level have no interaction, and are not related other than in the ability flavor.