Rib knit is a type of knit fabric created using two needles that has vertical textured lines. The vertical ribs are created with a certain number of knit stitches (more prominent) and a certain number of purl stitches (the groove between the ribs), repeated multiple times along the width of the fabric (which is usually made and sold in circular pieces, without any selvedge).

Depending on how many knits and purls, you can have different rib knit fabrics. A 2x2 rib knit will have a sequence of two knits and two purls.

Ribbed knits have wales or vertical rows of stitches that form ribs on both the face and the back of the fabric making both sides appear the same.  Ribbed knit are usually 100% cotton, but can also come with spandex and other fiber blends, and have a natural stretch that make them very useful for cuffs, bands, and necklines.  Rib knits are also popular for use in infant wear and tops and dresses.

Characteristics of rib knits:
It has a lot of crosswise stretch, even without any spandex content.
It usually recovers pretty well after being stretched.
Right and wrong sides are similar, but different: You can use either of the two as the right side, but choose one and stick with it.
It's stiffer than jersey and less smooth
When pulled, its edges don't curl like a jersey.
It perfectly hugs the body, highlighting shapes and curves.