Human mobility is subject to collective dynamics that are the outcome of numerous individual choices. Smart card data which originated as a means of facilitating automated fare collections has emerged as an invaluable source for analyzing human mobility patterns. A variety of clustering and segmentation techniques has been adopted and adapted for applications ranging from passenger demand market segmentation to the analysis of urban activity locations. In this paper we provide a systematic review of the state-of-the-art on clustering public transport users based on their temporal or spatial-temporal characteristics as well as studies that use the patter to characterize individual stations, lines or urban areas. Furthermore, a critical review of the literature reveals an important distinction between studies focusing on the intra-personal variability of travel patterns versus those concerned with the inter-personal variability of travel patterns. We synthesize the key analysis approaches and based on which identify and outline the following directions for further research: (i) predictions of passenger travel patterns; (ii) decision support for service planning and policy evaluation; (iii) enhanced geographical characterization of users' travel patterns; (iv) from demand analytics towards behavioral analytics.