Often referred to as sealant, caulk comes in cartridges for use in a caulk gun or in smaller tubes that are squeezed by hand. The best caulk to use to seal air leaks is silicone, as it stays flexible for a long time (50 years according to some manufacturer's guarantees). Silicone cannot be painted, however. If you must paint, use an acrylic latex caulk with silicone.

Even if windows and doors are sealed with weatherstripping, they can still allow a copious flow of air through gaps between the rough opening and the window or door jambs.

The best way to seal these gaps is to remove the interior trim and fill up the space with non-expanding foam. Remove the trim carefully by prying it up with a wide-blade putty knife and a pry bar.

Trim off excess foam with a utility knife and re-install the trim. A simpler but, also more visible, alternative is to leave the interior trim in place and seal around it with a clear silicone caulk or paintable latex caulk with silicone.