There are a handful of differences between the coil or "spiral" CFLs and the CFLs that resemble a traditional lightbulb (A-line shape). The first difference is the amount of light each will produce. Most times, a CFL that looks like an incandescent light bulb is really the "coil" shaped CFL with a plastic or glass cover. This cover will slightly reduce the amount of light that is produced. If you compared a 14W bare spiral CFL and a 14W "incandescent" shaped CFL, the bare product will provide more light for the same wattage. Also, bare CFLs usually have longer lifetimes than covered products.
Second, most bare spiral CFL products will perform like incandescent light bulbs - they will turn on instantly and provide full brightness. Covered CFLs may take slightly longer to reach full brightness
The last difference is the price - covered CFLs generally cost slightly more than bare spiral because of the additional materials required to manufacturer the products.