1. A computer system for iterative set expansion using samples, the system comprising: a processor and memory configured to execute software instructions embodied within the following components; an input component that receives a set of seed terms and a set of terms and associated contexts with which to expand the set of seed terms; a data modeling component that models the received terms and seeds as a bipartite graph with candidate terms being nodes on one side and identified context nodes on the other side, and models web query log data by dividing each query into a context of a fixed number of tokens of prefix or suffix and a remaining term; a similarity determining component that determines a similarity metric between two candidate nodes in the graph based on the candidate nodes' relationship to the context nodes in the graph; a relevance determining component that determines a relevance metric that indicates how similar a node in the graph is to the received seed terms and corresponding nodes; a coherence determining component that determines a coherence metric that indicates how consistent a concept set is that includes the seed terms and one or more candidate nodes; a quality measurement component that combines the determined relevance metric and coherence metric to determine a quality metric that indicates relevance and coherence among a set of nodes in the graph; an iterative expansion component that identifies an expanded seed set having a high quality metric; and a set reporting component that reports the identified expanded seed set as output.