How to highlight all occurrences of a word in an Emacs buffer?

The hi-lock suggestions are good. I think it's easier to use the M-x versions, though:

M-x highlight-regexp RET <REGEXP>

M-x highlight-phrase RET <REGEXP>

highlight-phrase is just a bit of sugar around highlight-regexp that ignores case and translates a space in the regex to match arbitrary whitespace. Handy.

Maybe highlight-symbol.el at http://nschum.de/src/emacs/highlight-symbol/ is what you are looking for: