Most art is considered subjective, but some writing we can all agree is terrible. (No, we're not talking about Eragon.) Horrible writing is, in general terms, taken to include horrendously urple prose or else no descriptions at all, poor sentence construction, poor spelling, punctuation, and grammar, overuse of Olde English (especially abuse of the second person familiar), and excessive chatspeak. It can also include badly formatted stories where everything is lumped together into one undifferentiated mass. These are usually impossible to read.
The first kind of horrible writing is treatable: a good beta reader or even a half-decent spellchecker can salvage such a story. The second kind of horrible story is a little more difficult to bring back, since the horribleness is so heavily entrenched. At some stages, it's too far gone to salvage and it would be easier to start anew.
A classic example of horrible writing is The Eye of Argon by Jim Theis. It contains some of the urplest prose known to fantasy fiction, malapropisms aplenty, and cat-on-the-keyboard names in abundance.