It's actually pretty similar except that they changed numbering when they run out of digits in some series.

For the current lineup:

• One-digit DSLRs are top-of-the-line full-frame cameras. The higher the number the newer. So D4 is newer than D3. There are sometimes variants such as D3S which is specialized for low-light and D3X which is specialized for high-resolution.
• Three-digit DSLRs are semi-professional cameras, both APS-C crop and Full frame. These include the D800 which also has a D800E variant which lacks an anti-alias filter but is otherwise identical, and the older D700. There is one current APS-C model in this series, the D500. Again here, higher numbers are newer.
• Four-digit models have cropped sensors (APS-C). There are three sub-series here. The semi-professional D7xxx, the basic D5xxx series and the entry-level D3xxx series.

For the older lineup:

• 2-digit models where lower meant more basic and higher numbers were better. So a D40 to D60 was entry level and a D70 to D90 was mid-range. These were the last models of their series as the numbers had no where to go from there.
• Before the D3, all Nikon DSLRs with APS-C. This included large professional models like the D2S and D2H.