I'm going to jump to the conclusion here that when you say "abs", you mean the core musculature in its entirety.
While Pancake's answer is good, it does only address part the story.
The core musculature does have several functions, flexion (as in a crunch), anti-flexion (when you brace such as in a squat or deadlift), extension (superman style exercise), anti-extension (a plank exercise), lateral flexion (side bends), anti lateral-flexion (1 arm farmers walks / suitcase carries), rotation (cable core rotation thing) and anti-rotation (Pallof press).
Now arguably, it's the anti exercises that are more important for proper spinal alignment and health than the flexion / extension exercises, as in life you're more likely to need to brace yourself against forces through your core than the opposite (picking up shopping from the floor, lifting children (preferably yours) overhead, holding a door for someone), though I have also read from some highly regarded strength coaches that flexion type exercises help lubricate and provide nutrients to the spine.
What I think pretty much all coaches agree on is that there's little point in doing thousands of crunches a day in pursuit of a six pack. Though I don't have time to search for it at the moment, from memory the rectus abdominis (six pack muscle) is made up from roughly equal parts fast twitch and slow twitch muscle fibres, so it's a good idea to mix up heavy, strength based crunch style exercises with more endurance type ones.
Basically, work everything, but don't go too nuts in any single direction.
