If there are words slightly to the right of the avatar immediately to the left of this sentence there's a decent chance it is an example of snark. But not always. Sometimes I'm helpful.
You and I both, buster!
To the OP: I think "snarky" is just a word that bored and hormonal internet women came up with to replace and place extreme emphasis on the word "sarcasm". "Snarky" replies can also be antagonistic in nature.
I never even heard of the word before I joined a wedding and a married life forum.
And if you'll take notice, I'm willing to bet that 90% of the people using that word on this forum are women... or men who got the term from a woman.