Often includes activities that would be included in "making out", e.g., necking.
One wag defined it as "anything you can do with a married woman in her husband's presence." (This assumes a 'monogamous' marriage rather than an "open" one.)
More general: Helen and Joe were sitting on the couch, snogging. She left lipstick all over him from the collar up. I swear his hands were inches from her tits. And Fred (Helen's husband) just sat there, watching the game on TV, completely ignoring it.
2. verb; to play tonsil hockey
3. verb; to give comfort or show affection to a fellow user of an internet forum. (syn: /lick, /spamHug)
"...let's snog, baby."
"Awww, sorry to hear that Joslo. /snog."