A helix is a curve in three dimensions. The thread of a screw is a helix, and wire coiled around a rod makes a helix. A spiral staircase is a helix. Some plants, like in the picture, naturally grow into helixes. The word "coil" is sometimes used to mean a helix and sometimes used to mean a spiral, or sometimes used to mean the act of wrapping something around something else to make a helix or spiral.