There are various types of explorers, though the most common one is the No. 23 explorer, which is also known as a "shepherd's hook". Other types include the 3CH (also known as "cowhorn" or "pigtail") and No. 17 explorers, which are useful for the interproximal areas between teeth.
The Tufts 17/23 Explorer, also known as the Wilken's Explorer, contains both No. 17 and No. 23 on the same instrument; opposite sides of each other.