Muslin Fabric History
Muslin use to be just the word for Fabric back in the ancient times. You could find it in a variety of weights and qualities from India's fine almost transparent delicate fabrics, to thicker and sturdier weaves.
Muslin is now typically referred to as a cheap or affordable cotton plain weave fabric. It's either bleached or un-dyed cotton, and is most often used for mocking up garments, testing your fit, or using as interfacing.
Good quality muslin can be used as a quilt backing, or quilting fabric, its dye-able and very affordable for beginner quilters.