If your lamp has a harp (the wire that attaches to the lamp and goes over and around the light bulb), you will need a spider lamp shade. The term "spider" refers to how the shade attaches to the lamp, which is known as the fitting. Spider lamp shades have the most common type of fitting for both floor and table lamps.
Spider lamp shades are easily identified. Look at the metal fitting inside the shade. If you see at least three spider-like arms radiating from a central ring toward the top, it's a spider shade. The ring fits over the top of the harp and is secured with a screw-on cap. These caps are called finials and they are often decorative and may match the lamp base.
If the ring is lower (toward the middle of the shade), it's probably an UNO fitter. UNO fitters secure to the socket at the base of the bulb, rather than over the harp. If you see a couple wire loops at the center of the fitting, you're likely looking at clip-on fitter. These fitters are handy if you lose a harp or don't have an UNO shade. They simply slip over the light bulb, so you should avoid clip-on fittings for heavy shades.