What is the difference between step-up and step-down transformers?

Step-up transformers increase voltage from primary to secondary (secondary voltage higher than primary), having more turns on secondary winding. Step-down transformers decrease voltage (secondary voltage lower than primary), having fewer secondary turns. Power remains same (ideally) in both: if voltage increases, current decreases proportionally. Step-up transformers are used at generation plants; step-down at substations for distribution and at consumer premises.