Differentiation is not just giving students different work. Instead it is observing students in a meaningful way and knowing their strengths and weaknessess. In turn, the teacher can provide enriched learning experiences to meet the needs of every individual student.
What is formative assessment?
Formative Assessment is NOT for a grade. It's purpose is to provide feedback to the teacher DURING instruction to help guide and adjust instruction accordingly. In addition, it is used to help students improve their work BEFORE they receive a grade. This type of assessment should be taking place daily and the teacher should modify or adjust instruction based on student work and responses.