Chinese celery (keun chai in Thai) looks a bit anemic when compared to the western variety. The stalks are thin but the tips are quite leafy. A bunch could be mistaken for coriander or parsley. But despite its lightweight appearance, Chinese celery is quite flavorful. Clear soups made with the celery look quite plain but are redolent with the taste of celery. The Thais have a large variety of clear soups, called gaeng jeud in Thai, which are quite popular among the lunch-time crowd at the curry stall. Almost every recipe for any clear soup calls for some Chinese celery. It's the key flavor that most of these soups will have.
The plant is not directly related to the European variety. Instead it evolved from a wild celery native to Asia. It seems to have come into wide use in Chinese cuisine around the fifth century A.D.