Teak is an oily wood that comes from the teak tree (Tectona grandis), which grows in Southeast Asia. It has a honey-brown color, and the natural oils migrate to the surface to give the fresh wood a shiny appearance. Because these oils protect the wood, teak is a preferred material for boats and outdoor furniture. Although it turns gray with exposure to sunlight and weather, the wood doesn't deteriorate, and the only maintenance it requires is periodic washing with soap and water. If you want to preserve the original color, though, you need to seal the wood before discoloration happens.
Refrain from putting any kind of protective coating at all on teak -- that's the easiest approach. The protective oils remain in the wood even after the surface turns gray, and you can always restore the color, if desired, with a thorough cleaning and sanding.
Apply a coat of synthetic teak oil with a paintbrush before the wood discolors if you want to prevent discoloration. Unlike organic teak oil, synthetic oil won't promote the growth of mold that can cause black splotches to appear on the surface.
Maintain the color by recoating the furniture with synthetic oil every few months. Wash the wood thoroughly with all-purpose detergent and water and let it dry before you recoat it.
Restore the color on weathered teak by first scrubbing it with an abrasive pad and scouring powder and water. If you need a stronger detergent, use a wood cleaner that contains oxalic acid, which will bleach out dark splotches.
Sand the wood after you clean it, using a pad sander and 120-grit sandpaper. Sanding will finish the job of opening the grain and restoring the color. Maintain the color by applying synthetic teak oil.