For you, newbie in DSLR and having Nikon D40 or D40x (like mine). This list might be helpful for your guidance to find compatible auto focus lenses for your camera :)
Nikon D40x
Picture courtesy of kenrockwell.com
Nikkor:

  • AF-S DX 12-24mm 4.0G ED-IF
  • AF-S 17-35mm 2.8D ED-IF
  • AF-S DX 17-55mm 2.8G ED-IF
  • AF-S DX 18-55mm 3.5-5.6G ED II (kit lense)
  • AF-S DX 18-70mm 3.5-4.5G ED-IF
  • AF-S DX 18-135mm 3.5-5.6 ED-IF
  • AF-S VR DX 18-200mm 3.5-5.6G IF-ED
  • AF-S 24-85mm 3.5-4.5G IF-ED
  • AF-S VR 24-120mm 3.5-5.6G ED-IF
  • AF-S 28-70mm 2,8 IF-ED
  • AF-S DX 55-200mm 4-5.6G ED
  • AF-S VR 55–200mm 4–5.6G IF-ED (my zoom lense)
  • AF-S VR 70-200mm 2.8G ED-IF
  • AF-S VR 70-300mm 4.5-5.6G
  • AF-S 80-200mm 2.8 ED-IF
  • AF-S VR 105mm 2.8G
  • AF-S VR 200mm 2.0G ED-IF
  • AF-S VR 200-400mm 4.0G ED-IF
  • AF-I 300mm 2.8D IF-ED
  • AF-S 300mm 2.8D IF-ED
  • AF-S 300mm 4.0D ED-IF
  • AF-S VR 300mm 2.8 ED-IF
  • AF-I 400mm 2.8 D IF-ED
  • AF-S 400mm 2.8D ED-IF
  • AF-I 500mm 4.0D IF-ED
  • AF-S 500mm 4.0D ED-IF
  • AF-I 600mm 4.0D IF-ED
  • AF-S 600mm 4.0D ED-IF

DX = for Nikon DX Format Digital SLRs VR = Vibration Reduction. IS (Image Stabilizer) in Canon ED glass = Extra-low Dispersion IF = Internal Focusing G = The G-type NIKKOR has no aperture ring; aperture should be selected from camera body D = Distance Information Sigma:

  • 10-20mm 4-5.6 EX DC HSM
  • 12-24mm 4.5-5.6 EX DG HSM
  • 14mm 2.8 EX
  • 17-35mm 2.8-4.0 EX DG
  • 17-70mm f2.8-4.5 DC Macro HSM
  • 18-50mm 3.5-5.6 DC HSM
  • 18-50mm f2.8 EX DC Macro HSM
  • 18-200mm f3.5-6.3 DC OS
  • 30mm 1.4 DC
  • 50-500mm 4.0-6.3 EX DG
  • 50-150mm 2.8 EX DC HSM
  • 55-200mm 4-5.6 DC HSM
  • 70-200mm 2,8 EX DG Makro
  • 80-200mm 2.8 EX DG Makro
  • 80-400mm 4-5.6 EX OS
  • 100-300mm 4.0 EX DG
  • 120-300mm 2.8 EX DG
  • 150mm 2.8 EX DG MAKRO
  • 180mm 3.5 EX DG MAKRO
  • 300mm 2.8 EX DG APO HSM
  • 300-800mm 5.6 EX DG APO
  • 500mm 4.5 EX DG HSM APO
  • 800mm 5.6 EX DG APO

HSM = Hyper-Sonic Motor, This lens uses a motor driven by ultrasonic waves to provide a quiet, highspeed AF OS = OS (Optical Stabilizer) Function EX = EXpensive hehehehe :P The exterior of this lens is EX-finished to denote the superior build and optical quality, and to enhance its appearance. APO = In order to attain the highest quality images, the APO lens has been made using special low-dispersion (SLD) glass and is designed to minimize color aberration ASP = The aspherical lens complex allows freedom of design, improved performance, a reduced number of component lenses and a compact size DG = The most suitable lenses for 35mm film single-lens reflex cameras, as well as for digital SLR cameras. Sigma’s development of the DG (Digital) range of lenses has concentrated on the correction of distortion and aberrations. Suitable for digital and analogue cameras DC = These are special lenses designed so that the image circle matches the smaller size of the image sensor of most digital SLR cameras