A grob is dug into the ground so that each half slopes downward to meet in the middle at the deepest point. It is usually the width of the rails used for the jumps, so it can be relatively narrow and some horses don't enjoy the feeling of being funnelled down into such a restrictive, low area. For this reason it can be an especially spooky jump.
At its most difficult, the grob is a triple combination with one or two strides between each jump, depending on the length of the particular grob. It's more difficult at one stride because there's no room for error if the horse backs off on the way in. The course designer can also choose to only place a single jump in the center of the grob for the easiest set-up so that there's no striding to worry about.
There is usually a dry ditch or liverpool located at the bottom of the grob, under the middle jump. This can make the horse back off the combination even more.
Most grobs are dug straight, but some, including the one I am showing in this photo (with no jumps set up in it), are curved.
Due to the nature of the obstacle, a refusal at the second or third obstacle in the combination will require the horse to circle and retake the jump using only the space between that jump and the previous jump. Because the sides rise up as the horse goes down into the grob, there is no way for the horse to exit out the side to retake the combination as a whole.