Difference between "heck" and "hell"
Solution 1:
"Heck" is a euphemism for those who feel the word "hell" constitutes strong or inappropriate language.
Solution 2:
Although I believe heck is more slangy than hell, it might be considered as an euphemism for hell or, most likely, a fusion of hell and f***.
Whether it's what the hell or what the heck, neither of them should be used in a formal or respectful conversation.
Solution 3:
Expletives like "Heck!" are minced oaths for "Hell!" according to our Sunday School teacher back in the days when such things mattered.