纤毛虫是一类较复杂的原生动物，主要特点是以纤毛作为运动器，细胞核一般分化出大核（营养）、小核（生殖）、摄食胞器等，无性生殖为横二分裂，有性生殖为接合生殖，生活在淡水或海水中，也有寄生的。代表生物有草履虫，小瓜虫等。

纤毛虫在分类上比较复杂，尚无统一的定论。按传统的分类，原生动物作为动物界中的一个门，纤毛虫为其中的“纤毛纲”（Ciliata），有时也将其列为原生动物门的纤毛亚门，或将原生动物看作动物界的一个亚界，纤毛虫划为一个门——纤毛门。按照汤玛斯·卡弗利尔-史密斯提出的分类系统则属于囊泡藻界的“纤毛虫门”（Ciliophora）。

The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar Suctoria only have them for part of the life-cycle) and are variously used in swimming, crawling, attachment, feeding, and sensation. All behavioral patterns are coordinated by signaling processes. Nevertheless, although not a general practice, some authors prefer to call "cilia" both eukaryotic cilia and flagella, reserving the term "flagella" to the bacterial structure. Anywise, the fundamental difference between multiciliate flagellates (e.g., hemimastigids, Stephanopogon, Multicilia, opalines) and ciliates is the presence of macronuclei in ciliates alone.