Default classes are replaced with smith, soldier, farmer, handy.
You start with level 0 and gain expirience depending on class,
smith - from forging on anvil, mining ores
soldier - killing creatures
farmer - collecting mature crops
handy - treecutting, block chiselling

Bonuses:
smith - mining speed, ore drop, durability bonus to forged tools, can repair armor
soldier - hunger rate is decreased, more health points, armor loses less durability, weight bonus(if weight mod isntalled),
can create different bows, accuracy and ranged weapon speed bonus
farmer - more drop from crops, fruit trees;higher chance to get farmland block back on destruction; can seal crock N times
without fat in real day; tree seeds, sticks, woods bonus drop
handy - can copy N chisel blocks in real day; bonus drop for charcoal, clay, peat; can craft cloths

If you select one of the new classes and remove mod after that - default character system won't like value of player's class
and player will have exception and kicked from game. Maybe something changed and it now fallback to one of the default classes
but I didn't check it.