As stated in the introduction, TODO isn't a keyword, it's a comment. Its exact meaning will depend on the local coding culture and often individual programming style. For example, the Sun Java programming conventions state:

Already we've found a shocking new member of our TODO set: XXX!

I think it's clear, though, that the phrase TODO in a comment indicates something more is required. With any luck, the rest of the comment will indicate exactly what that something is.

FIXME often seems to be used interchangeably with TODO but carries a stronger suggestion that something is broken—if TODO is a feature request, then FIXME is a defect report. Again, with luck, the rest of the comment will explain what needs fixing.

Unfortunately the rest of the comment is often inadequate. Sometimes we'll find a bare:

// TODO

sometimes an initialled note-to-self:

// FIXME TAG

sometimes a plea for attention:

// TODO Fix this hack !!!

and sometimes even a garbled attempt to cover all bases:

// TODO FIXME XXX HACK

Of course, TODO isn't meant to be pretty. The capital letters shout at the reader, drawing attention to the deficiency. The noise will continue until something gets done.