GeoTopics is a system to explore geographical patterns of urban activity. The system collects publicly shared check-ins generated by Foursquare users, that reveal who spends time where, when, and on what type of activity. Specifically, each venue in the data is associated with the following features: an exact location, a category (e.g., restaurant or museum), the set of users who have checked-in there, and the time (time of day and day of week) they checked-in. The method at the core of the system is a sparse probabilistic modeling technique to learn associations between different regions of a city and multi-feature descriptions of urban activity. As an example, one might discover that the south of a city is strongly associated with restaurants, bars, and night-clubs that are active in the evening, while the north of the city is associated with cafeterias and parks that are active in the morning of weekends.