The Intel architecture model 80x86 designates each processor core. The X is a number from 0 to infinite but for now it is from 0 to 7. Each model adds few more instructions and sometimes a new feature although each model does not have to increase transistor count. If you have an 80686 processor, you can run software that was compiled for 80686 and lower. If you have an 80386, you can only run software compiled as 80386 and lower.