Nikon's entry level Digital Single Lens Reflex camera which has a 6.1 MP CCD sensor similar to that used in the Nikon D50, D70 and D70s. Introduced in Q4 2006. It weighs approximately 471g (without battery), which is 69g less than the D50, and is 133x102x76mm in dimension. There is a built-in flash and continuous shooting up to 2.5 frames per second. Images are recorded on an SD/SDHC card only, battery is Nikon's EN-EL9 Lithium-ion type. RAW shooting in Nikon Electronic Format is available as well as conventional jpg image format (fine, normal or basic as well as RAW+jpeg basic).
Several design considerations reduced the D40s size, weight and cost. The D40 uses a pentamirror, has no top LCD, no built in AF motor only three AF/Spot metering zones and no sensor cleaning features.
Several implementation details set it up as an entry level DSLR. There is no DOF preview, no mirror lockup and no exposure bracketing. Presumably all these features could have been implemented without much increase in cost, and the decision not to was a marketing one.
The D40 is considered and excellent camera especially for people who have not much experience with SLR cameras. For people who have an existing stable of Nikon F mount lenses a body with a built in AF motor would serve better.