- gh, pronounced /f/ as in tough /tʌf/;
- o, pronounced /ɪ/ as in women /ˈwɪmɪn/; and
- ti, pronounced /ʃ/ as in nation /ˈneɪʃən/.
To spoil all the fun, the problem with this is that the digraph "gh" is only pronounced /f/ at the ends of syllables, never at the beginning. Also "ti" is not a recognised English digraph and only can only be pronounced /ʃ/ before -tion -tian, -tious -tient, -tiate, i.e. never at the end of a word.
Ghoti is not the only "ghoti spelling" - "ghoughpteighbteau tchoghs" is also given as a creative way of spelling "potato chips".
As an element of the argument for spelling reform, its use is somewhat self-defeating. The effect hinges on the audience being surprised that ghoti would be pronounced "fish", which presupposes that they in fact have notions of the typical pronunciation of the orthography involved. An average English speaker who encountered this word would expect it to be pronounced /ˈɡoʊˌti/ ("goaty"), or similarly, and it is from precisely this reliability that the argument for unreliability gets its force.