Shellac is the brand name for a new, patent pending nail product created by Creative Nail Design (CND). It is a hybrid, meaning half nail polish, half gel. The product can be applied similarly to nail polish on your natural nails (no sculpting or filing). It is cured through UV lighting, just like Gels.
A gel manicure and a Shellac manicure are essentially the same thing: long-lasting polishes cured under UV lamps. The difference is that "gel nails" or "manicure" is the generic term for nails done under a UV lamp, while Shellac is a brand of gel nails by the company Creative Nail Design (CND).
About two weeks. Shellac was the first hybrid polish, produced by CND, and you can expect manicures to last about two weeks. Some may find that the chip-free finish lasts longer than this, but it's recommended that you get them removed after 14 days as by this time your own nail will have started to grow out.
In fact, experts claim there are several points in the gel/shellac process that not only have the potential to damage your nails and nail beds (acetone, anyone?), but could also put your health at risk. Overzealous nail filing to prepare the nail for layers of gel can expose new skin to chemicals and infections.
It is possible to ombre a shellac colours with a powder. For example, paint the whole nail in bicycle yellow and then blend a neon orange powder onto the top half of the nail.