Betweens / Quilters needles are generally a shorter needle than most (typically between 30 and 38mm long). They have a small rounded eye and are used for making fine stitches on heavy fabrics such as tailoring, quilt making and other detailed handwork.
As a general guide, needle sizing is referred to by its wire gauge number. The lower the number, the thicker the needle. For example a pack of 3/9 would be an assortment of 16 needles ranging between size 3 and 9. A pack of size 8 would be all 16 needles in size 8 and is quite a thin diameter needle compared to a size 3.