a lens with a fixed focal length

50mm f/1.8D - the cheapest lens but no built-in motor
35mm f/1.8G - slightly more expensive but got built-in motor

if you're using D3k, D5k, or those body without AF motor, then go for the 35mm. if you're using D80, D90 or bodies with AF motor then can go for 50mm.

D means there's a distance meter that feeds back the distance of subject to the camera. G doesn't