fire (18/51 + 2) is the top spec when you have best in slot gear for 25 man raiding (good for 10 man too) - you need 340-380 hit so its very gear dependent, its not a spec you want to do when in heroic/naxx10 gear, only when you are in good naxx25/ulduar gear

arcane (51/3/17) is the top spec for starting raiding, only needs 200-240 hit and has the highest burst (so great for very short fights). its very competitive with fire in 10 man raids, and can do well in 25 man on most fights, its just that you are very dependent on not getting evo screwed by having to move, and on ulduar boss fights can be long and require lots of movement. arcane is also a great PVP spec although not as good as frost

FFB (0/53/18) isn't that great a spec anymore due to the nerfs they did to stacking fire/frost buffs for ffb. no one should really be spec'd ffb anymore

frost is the best PVP spec, its a mediocore PVE spec. the max theoretical/realistic DPS is quite a bit below fire and arcane, but it has higher survivability, movement, and control (ie great for arenas)