Precuts come in a few convenient, standardized measurements. They should be the same dimensions from one fabric manufacturer to another, although the terminology for each may vary.
Jelly Rolls, also known as 2 ½ " strips, strip packs, strip-pies, and pixie strips, are 2 ½ " wide × width of fabric (WOF). A jelly roll quilt is named for the pre-cut strips that you use to make the quilt. These strips are usually arranged by color and come wrapped up in a roll. You can sew the strips together in the order they are in, or you can go through the stack and separate the colors a bit more.
A charm pack is a bundle of 5" squares of precut quilt fabric featuring an array of coordinating fabric from a designer's collection. Charm packs are a fast, fun way to make quilts come together like magic.
Layer Cakes & 10" Square Quilt Fabric are collections of 10" x 10" squares of fabric made by Moda Fabrics. They are similar to a charm pack but a larger size. Layer Cakes are available by collection and typically include 42 pieces of fabric, though the number may vary.
A fat quarter of fabric is a one-fourth yard cut that usually measures 18" x 22". The longest side's dimension sometimes varies because some quilting fabrics are not 44" wide.