What is the Difference between rising stem gate valve and non rising stem gate valve?

Rising stem gate Valve- Stem threads are out side of valve body and in no contact with fluid medium. Stem rises while opening the valve (by hand wheel). Since the stem threads are located outside obviously it needs some more vertical clearance space while opening the valve.

Non Rising Stem gate Valve - Stem threads are inside the the valve body and it is in contact with fluid medium. Hence, lot of chances for fouling the threads and these types of valves only can be used in clean liquids and gases. Less vertical clearance space required while opening the valve.