At low temperatures (say, as low as it gets on a winter night at the typical observing site near Bangalore), if the air is humid enough, the moisture condenses on any exposed surface, including mirrors, corrector plates, and objective lenses.
This is the dew problem. Its primary effect is that it can derail your observing session by giving the proceedings a foggy appearance.
That apart, the dew also exposes you to the problem of accumulating sticky dust on your telescope optics - especially if there are trees around your observing site.
Prevention is better than cure, and this is true of dew as well.
Since dew only forms on exposed surfaces, one solution is to shield them. This can be done by having a long enough shade, or dew shield extending from the objective end of the tube. This applies primarily to Refractors and Maks.
This Sky & Telescope article has more to say about dew shields, including how to make your own. If you’re looking for foam rubber (as mentioned in the article), all that you need to do is to get hold of a Yoga Mat.
Sanath’s article on dew heaters has references to sources for other things you might need, like Velcro strips.
Per a discussion with Sathya, the formula for the length of the dew shield is 1.5 x D + 50mm, where D is the telescope aperture in mm. Sathya also recommends using corrugated cardboard reinforced by a covering of duct-tape instead of a rubber mat.
Dew Shields are the first line of defence against dew. Sometimes it is cold enough for dew to form despite dew shields. What you then need is a dew heater. Think of it as a dew shield with a built-in heating mechanism.
Dew Heaters work by maintaining the temperature around the objective (and hence that of the objective as well) above the dew point. While commercial dew heaters are available, it is not too difficult to make your own.
Sanath has written up plans to make your own dew heater. It also comes with vendor/shopping references in Bangalore for the parts involved.
A discussion on the BAS google group might also make for interesting reading.