What's the difference between TWEBs and TIBS?
NOAA has a great guide on their website called A Pilot's Guide to Aviation Weather Services which covers these (and more)!  They actually describe it better than I could.

The short version though is that TIBS is a telephone weather service while TWEB is a recorded route briefing that you can listen to while airborne.  Both are used for supplemental information and don't replace an official briefing.


  
  TIBS Telephone Information Briefing Service  
  TWEB Transcribed Weather Enroute Broadcast  
  PATWAS Pilot's Automatic Telephone Weather Answering Service
  





  TRANSCRIBED WEATHER BROADCASTs (TWEB) are continuous broadcasts of
  recorded NOTAM and weather information prepared for a 50-nautical mile
  wide zone along a route and for selected terminal areas. TWEBs are
  broadcast over selected NDB and VOR facilities and generally contain a
  weather synopsis, in-flight advisories, route and/or local vicinity
  forecasts. Winds Aloft Forecasts, current weather reports, NOTAMs, and
  special notices. TWEB outlets are listed below by state:





  PILOT'S AUTOMATIC TELEPHONE WEATHER ANSWERING SERVICE (PATWAS), and
  the TELEPHONE INFORMATION BRIEFING SERVICE (TIBS), provide continuous
  recordings of weather and aeronautical information. The information
  may include area and/or route briefings, airspace procedures, and
  special announcements. Telephone numbers for PATWAS and TIBS locations
  are found in the Airport/Facility Directory.





  TWEBs, PATWAS, and TIBS are for preflight or inflight planning and
  should not be considered a substitute for formal preflight briefings.