Is there any web UI for talend in which we could define mappings from the WEB UI only.
In this way a user can provide mappings at run-time.
Unfortunately no by design. All development work is centralized in the Talend Studio a heavy IDE based on Eclipse. However in the Enterprise versions of Talend, mappings built in the studio can be managed by the WEB UI: Talend Admin Center. Any configuration fed into a mapping can be updated there without having to go into Studio.