Lemons are fanfics that have sex in them.  I've heard of Limes, I assume they
have something to do with sex, but I'm not sure.  Probably have less
descriptive sex scenes in them(limes arent as sour as lemons).  Not that I
know for sure, but it probably origonated from an older adult anime called
"Cream Lemon".  If I rember corectly, it was set a future high school(a-la
"Project A-ko" complete with mech mechanics class and such.)  And it featured
a number of diffent storylines.  The only episode I even know anything about
was titled "Pop Chaser", and featured a number of mecha fight scenes, along
with the 'other' thing that made the series famous...