Forward slash goes bottom right to top left. Backslash does the opposite.
Seriously though, neither has a formal meaning. Slashes are not considered
punctuation at all by Fowler or Garner, and are not mentioned except (can't
remember where) to castigate illiterate usages like "architect/engineer". In
almost all cases the sense is better expressed with 'or' or 'and' as the
case may be.
Backward slashes were never used at all, as far as I've ever seen, until
Microsoft started using them for path delimiters in MS-DOS; and that's still
their only common usage.
