Whats spark plug gap?

[tim21391]

ok before i changed my plugs...on mornings it was cold...i start up and i rev and its kind of like a delayed rev where i hit the gas and it takes a second for the engine to rev...then it backfired.....my friend works at autozone he said its fouling of the plugs....i go buy 8 new ones....he said to gap them at 35 thousandths....i do so and its backfiring worse...i look at the old plugs and they are at 40 thousandths...i put the new ones to 40 thousnadths....and it seems to be fine?? since its a further gap are those plugs going to mess up in a couple months?

thanks,
tim