A ghillie suit is a camoflauged suit, mostly used by snipers in the army. It usually has camo net, burlap, and other materials to help it blend in. The materials used will vary depending on the type of surroundings which one intends to blend in with, as seen here:
EDIT: These things look really suspicious in civilian settings, and are right up there with ninja suits when it comes to provoking “WTF are you doing?” questions.
EDIT: There are ghillie suits being developed lately that are able to conceal the wearer from FLIR (forward looking infrared) devices. The cost of these is prohibitive for most at around $3000 compared to $100-500 for a "regular" ghillie suit. These suits are also classified as munitions under US law and cannot be exported without a permit.
IF you are someone in a position to wear one legitimately. . . . . *You do not BUY a ghillie suit for $100. You MAKE your own ghillie suit. *It is made from an old pair of BDUS or an old flightsuit. You spend 100s of hours painstakingly sew pieces of netting and jute burlap to the suit with fishing wire.
*"FLIR" are most commonly called "thermals" or thermal sights. No ghillie suit is going to 'protect' you from being seen by thermals. Bodies make heat and dissipate it to your clothing. This is the biggest problem faced by the wearer of an ordinarily constructed ghillie suit - the temperature in the suit can reach well above 110/120 degrees.
You can indeed purchase a super-hightech suit from US Tactical Supply online - but uh. . . .why would you want to?