The terms Needlepoint and Petit Point (sometimes spelled Petite Point) are often used interchangeably, but are actually very different.
Petit Point is always comprised of very small, fine stitches and is often stitched on single threads of Penelope canvas or fine needlepoint canvas or Congress Cloth. The term has also been used to describe very fine cross stitch.
When worked on double-thread canvas (Penelope), it is often called the Quarter Stitch, especially in Europe. This refers to one stitch being worked over 1/4 of the usual stitch area - a single thread intersection rather than the double-thread intersection.
Stitching an area of Petit Point on Penelope canvas allows a stitcher to create greater detail in an area of the design. Instead of one (1) stitch at an intersection of Penelope canvas, the intersection is split into four (4) smaller stitches.
In the photo shown, you can see an example of Petit Point over single threads in the Penelope canvas (center area), versus standard needlepoint (border area).
NOTE: Tent stitch is also often called Gros Point or Demi Point when worked with in conjunction with Petit Point.