Differentiated instruction is a proactive way to educate. Teachers that use differentiated instruction know that students learn differently, assume they have different educational needs and plan accordingly. Classroom time isn't wasted, as teachers know from the very beginning that they are going to focus on meeting the needs of individuals, not entire classrooms.
Assessment is an important part of differentiated instruction. By assessing each student, the teacher determines who needs help and who is ready to progress to the next level. Without the proper use of assessment tools, teachers would just have to guess which students need what type of instruction. Assessment doesn't just happen at the beginning and end of a unit of study, it is ongoing. It also is sometimes informal, possibly using teacher observation to determine the type of instruction a student needs.
A teacher using differentiated instruction will use a variety of methods to teach a lesson. This means that not all students will complete the same assignment. Teachers may take into account not only ability, but personalities and personal preferences, as well. For example, some students may thrive when they're allowed to approach a task independently, while others need much more instruction. The students that prefer independent work may work quietly on reading a piece of literature while the other group of students participates in a teacher-led discussion.
Flexible grouping is vital to the success of differentiated instruction. Students placed in a group at the beginning of the school year won't remain there until the end of the school year. Instead, grouping is fluid and changes as the needs of the students change. There are times when it's necessary to group students according to ability level and times when it's important to group them according to learning styles. Students are usually very good in some areas and struggle in others, so their groups may change depending on how well they understand a particular concept.
Quality is much more important than quantity in differentiated instruction. For example, if differentiated instruction is used in a math class, kids that struggle with a concept won't simply be assigned twice as many math problems to complete. Rather, the kids that are struggling may work in a small group with the teacher or learn a different approach to solving the math problems.