1. A computer-implemented method of generating concept units from user search queries, the method comprising: receiving a plurality of queries, each query comprising a string of one or more words; tokenizing each query string to produce one or more tokens for each query, wherein said tokens for said queries form an initial set of units; combining units from the initial set of units that appear adjacent each other in a query to form a second set of units; validating the second set of units; repeating the steps of combining and validating one or more times using the second set of units in place of the initial set of units until a convergence condition is satisfied, wherein a final set of units is formed once the convergence condition has been satisfied; and storing the final set of units to a memory.