The 'code' as used to describe model railroad track refers to the height of the rail in decimal inches.  Code 100, which is the most common type, has rails that are .100 inches high.  Code 83 has rails that are .083 inches high while code 70 has rails that are .070 inches high.  Once you get into N scale, code 55 has rails that are .055 inches high.  Code 70 is the closest to actual scale size in the world of HO, although it is harder to find than code 83 or code 100.   We recommend rolling stock with metal wheels only for code 70 & code 83, as cars with plastic wheels have a tendency to derail.  Then again, we recommend cars with metal wheels for superior tracking regardless of the track code you use.