Jersey is a knitted fabric and from what most basic T-shirts are derived. It is knitted in rows, which means it will have more stretch in either a horizontal or vertical fashion. A mesh backing or medium-weight cutaway backing is needed to help stabilize the material when embroidering.
Source: Ed Levy
The Jersey knit, also called plain knit, is generally used for basic T-shirts material. It produces a smooth material with a fine pattern of vertical (wales) lines on the outside and horizontal rows (courses) on the side.
Source: The Encyclopedia of Garment Printing