A "ground pole" is typically placed at the base of a jump to give the horse/rider a sight line to judge take offs. A "placing pole" is a pole placed in front or behind the jump at varying distances to help fix a problem the horse or rider may be having. The terms are interchangeable, if you have a ground pole a couple strides away from a jump it's also a placing pole, it really depends on the lingo used in your area.
Rails are used in front or behind jumps to help correct problems. Rushing, landing in a heap, or rider timing are just a few.
The distance from the jump for placing poles varies according to what you are trying to accomplish. A horse that rushes a fence may need a placing pole closer to the jump, where a horse that lands in a heap after a jump may need more space on the backside to encourage him to lengthen away from the jump.