what is an AIS lens?

Answer 1

AIS lenses are old manual focusing Nikon lenses. The AI stands for Automatic Indexing (introduced in 1977) and Nikon uses the letter 's' to indicate an improvement (Nikon refined the AI design in 1983).
For an in-depth and fairly technical explanation, see: http://www.kenrockwell.com/nikon/nikortek.htm#ais
Nikon AI-s lenses can be used on all Nikon (digital) SLR cameras but you'll experience limitations with entry level models. With Nikon's current digital SLR cameras, AI-s lenses are crippled on the D40, D50 and D80 (no metering), and fully functional on the D200 and D2Xs.