Formative assessments are usually informal and not worth a grade. Summative assessments are usually much more formal and worth a grade.

Formative assessments are continuous assessments throughout a unit that are used for students to understand where they need improvement in their learning and for teachers to understand where they improvement in their instruction. Summative assessments are qualitative assessments given at the end of a unit that are used for students to gauge progress in their grade-level goals and for teachers to gauge whether or not the student is ready to move on with their education. 

In my experience, formative assessments are usually informal and not worth a grade. Summative assessments are usually much more formal and worth a grade.