Formative Assessments
A formative assessments is something the teacher does throughout the lesson to make sure their students are understanding the concepts. After gathering this data through small checkpoints, the teacher can then go back and change the lesson plans. If half your students still aren't understanding an idea, then you should probably spend one more day on it to try and get more understanding from your students.
Performance Tasks
A performance task is task or test the students do at the very end of a unit to show their understanding of what was being taught. This only happens once throughout a unit to ensure that the student fully understands. This one is always a tricky one to be able to identify. The task doesn't always have to be "performed" and that's what throws some people off when identifying this task.
Think of formative assessments as the quizzes you do throughout the lessons and the performance tasks as the test you do at the very end of the unit.
Formative Assessment Example:
"Class if you understand how to properly use a microscope, give me thumbs up. If you are unsure, put your thumb in the middle, and if you need some more practice, thumbs down."
Performance Tasks Example:
"Okay for your final task, I want you to properly use this microscope to find a single cell organism. Once you find it, keep it there, and I will be by to check."