As proper nouns the difference between cassie and jeff

is that cassie is a diminutive form of the female given name Cassandra while Jeff is a diminutive of the male given name Jeffrey or Jeffery.

As a noun cassie

is a thorny shrub with fragrant yellow flowers, species: Vachellia farnesiana; the needle bush.

As a verb jeff is

to play or gamble with em quads, throwing them as dice.