What is the difference between a ring counter and Johnson counter?

A ring counter circulates a single 1 (or 0) through a shift register; for n flip-flops, it has n states and counts from 0 to n-1. A Johnson counter (twisted ring) connects the complement of the last flip-flop output to the first input, creating 2n states for n flip-flops. Johnson counters are more efficient but require decoding logic. Ring counters have simpler decoding (one flip-flop per state) but are less efficient. Both are self-correcting in proper designs and provide glitch-free outputs.