Knitted fabric has it own special qualities, these are made by machines that take the threads and interlock looped stitches just as with hand knitting.
Depending on the tightness of the knit, these can be stretchy, they are comfortable to wear and easy to sew as the ends do not fray as with woven fabrics.
They also tend not to crease and come in many different weight from sheer lingerie to very heavy jumpers.
These do what their names suggests. They don't stretch much and can handle like woven fabrics. Double knits are in this group and it can be hard to tell the right or wrong side of a fabric especially if they have lengthways ribs on both sides.
Rachel knits don't stretch and can be bulky so is used for coats, jackets, dresses and skirts. These knits can also be used outdoors and for backpacks.
These knits have a lengthways rib on the right hand side of the fabric and loops going crossways on the back. These stretch slightly diagonally rather than lengthwise. A fabric made like this would be a jersey tricot.
These are either single or double knit and can be distinguished from their look. The fabric has a texture on the right hand side. Fabrics made like this are Terry or velour.
These have lots of stretch both diagonally and lengthwise. Sportswear is made like this from cotton/lycra, polycotton/lycra mixes.
The added lycra is a synthetic fibre that stretches and this gives the knit fabric the ability to stretch but also more important the ability to regain its original size.
These knits are lightweight, quick to dry and comfortable because they can stretch with the body movements.
Fibres used for these are nylon, cotton, polyester cotton, polyester and polypropylene each brings different qualities to the fabrics they produce, such as absorbency, heat retention and water repellant.
These knits are very stretchy and used for wrist, waist, neck and ankles. This can be bought as a trim for finishing garments.
A firm knit fabric can be made into cushions but probably only scatter cushions as the fabric could stretch out of shape if used on a base cushion seat.
Cushions of a different shape with curves etc would work well with a knit based fabric to give a smoother finish as the stretch of the fabric would hug the cushion insert.
Whatever type of fabrics you choose enjoy your cushions.