"b/w" is used to abbreviate three different expressions:

  1. "backed with". Commonly used in musical contexts to refer to the B-side of a 45 or 78 RPM record, or of a cassette.
    For example, a U-Men single featuring "Freezebomb" on the A-side and "That's Wild About Jack" on the B-side could be described as "'Freezebomb' b/w 'That's Wild About Jack'".
  2. "black-and-white". Commonly used in artistic contexts to refer to visual art works that have been produced in monochrome.
    Eraserhead, "Dizzy Dishes" (the first appearance of Betty Boop), and Depression-era photographs are all examples of b/w works.
  3. "between". To my knowledge, this use is context-independent, though you're unlikely to see it in the above contexts.

The abbreviation also appears in Homestar Runner and Two More Eggs, web series by Matt and Mike Chapman. Here, the abbreviation is (with rare exceptions) used instead of a more appropriate conjunction or preposition. (e.g. "pork b/w beans", "arts b/w crafts", "Zesty b/w tangy!")