Grass and the soil held by the roots. Also known as 'sod'.
Turf is the grass (typically lawn grass) and the top few inches of soil that are held together by the grass roots. "Turf" is used to refer to lawn grass, whilst "sod" is a more general agricultural term.
Turf can be purchased and used as a "lawn transplant". This can be a useful short cut to producing a mature lawn, but it requires a lot of maintenance work to make it worthwhile compared to the much cheaper process of sowing grass seed.
Use this tag for all questions about turf maintenance, and turf application.