14. The apparatus according to claim 10 , wherein the user is one of a plurality of users, wherein the interface is configured to communicate with the plurality of users, wherein the desired level of personalization includes the community level, and wherein the processor is configured to receive the indication of the community level; construct a topic profile that represents interactions of a plurality of previous searches conducted by the plurality of users within a single topic, wherein each of the previous searches includes one or more previous query terms; and select at least one of the previous query terms for inclusion in the set as one of the refinement terms, responsively to a level of association of the previous query term with the first query terms in the topic profile.
14. The apparatus according to claim 10 , wherein the user is one of a plurality of users, wherein the interface is configured to communicate with the plurality of users, wherein the desired level of personalization includes the community level, and wherein the processor is configured to receive the indication of the community level; construct a topic profile that represents interactions of a plurality of previous searches conducted by the plurality of users within a single topic, wherein each of the previous searches includes one or more previous query terms; and select at least one of the previous query terms for inclusion in the set as one of the refinement terms, responsively to a level of association of the previous query term with the first query terms in the topic profile. 15. The apparatus according to claim 14 , wherein the topic profile includes a topic association graph (TAG), and wherein the processor is configured to construct the personal profile by constructing the TAG including at least a portion of the previous query terms as vertices, and to select the previous query term for inclusion in the set responsively to finding that the previous query term is directly linked to all of the first query terms in the TAG.