Ropewalking is when the horse puts 1 foot in front of the other. Picture how a tight rope walker walks - that is the same fashion. Typically this is seen in the front end, it's less common in the hind.
Stringhalt is a sudden flexion of one or both hind legs in the horse resulting in a very exaggerated high stepping gait, it is most easily seen while the horse is walking or trotting. It is most evident when the horse is backing up slowly, turning on the affected leg, or suddenly frightened. It can involve one or both hind legs of the horse.
Tying-up is a broad term that frequently is used to describe a wide variety of muscle disorders that affect the performance horse. Other names given to this syndrome include exertional rhabdomyolysis, Monday morning disease, and azoturia.