Why is AC more "dangerous" than DC? After going through several forums, I became more confused whether it is DC or AC that is more dangerous. In my text book, it is written that the peak value of AC is greater than that of DC, which is why it tends to be dangerous. Some people in other forums were saying that DC will hold you, since it doesn't have zero crossing like that of AC. Many others also say that our heart tries to beat with the frequency of ac which the heart cannot support leading to people's death. What is the actual thing that matters most?
After all, which is more dangerous? AC or DC?
 A: The RMS (root-mean square) value of an AC voltage, which is what is represented as "110 V" or "120 V" or "240 V" is lower than the electricity's peak voltage. Alternating current has a sinusoidal voltage, that's how it alternates. So yes, it's more than it appears, but not by a terrific amount. 120 V RMS turns out to be about 170 V peak-to-ground.
I remember hearing once that it is current, not voltage, that is dangerous to the human body. This page describes it well. According to them, if more than 100 mA makes it through your body, AC or DC, you're probably dead. 
One of the reasons that AC might be considered more dangerous is that it arguably has more ways of getting into your body. Since the voltage alternates, it can cause current to enter and exit your body even without a closed loop, since your body (and what ground it's attached to) has capacitance. DC cannot do that. Also, AC is quite easily stepped up to higher voltages using transformers, while with DC that requires some relatively elaborate electronics. Finally, while your skin has a fairly high resistance to protect you, and the air is also a terrific insulator as long as you're not touching any wires, sometimes the inductance of AC transformers can cause high-voltage sparks that break down the air and I imagine can get through your skin a bit as well.
Also, like you mentioned, the heart is controlled by electric pulses and repeated pulses of electricity can throw this off quite a bit and cause a heart attack. However, I don't think that this is unique to alternating current. I read once about an unfortunate young man that was learning about electricity and wanted to measure the resistance of his own body. He took a multimeter and set a lead to each thumb. By accident or by stupidity, he punctured both thumbs with the leads, and the small (I imagine it to be 9 V) battery in the multimeter caused a current in his bloodstream, and he died on the spot. So maybe ignorance is more dangerous than either AC or DC. 
A: If you have a dc voltage of $x$ volts, this is the maximum voltage you can get from it.
If you have an ac voltage of $x$ volts, the maximum voltage is more than $x$ (I forgot how to calculate it, maybe it $x\sqrt2$, someone correct me if I'm wrong). This is because the voltage rating is the average of the oscillating voltage (after taking all positive).
And, a higher voltage means more dangerous, right?
EDIT: Check this wikipedia example out: http://en.wikipedia.org/wiki/Alternating_current#Example
A: I think that this page explains it very well: http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

Direct current (DC), because it moves with continuous motion through a
  conductor, has the tendency to induce muscular tetanus quite readily.
  Alternating current (AC), because it alternately reverses direction of
  motion, provides brief moments of opportunity for an afflicted muscle
  to relax between alternations. Thus, from the concern of becoming
  "froze on the circuit," DC is more dangerous than AC.
However, AC's alternating nature has a greater tendency to throw the
  heart's pacemaker neurons into a condition of fibrillation, whereas DC
  tends to just make the heart stand still. Once the shock current is
  halted, a "frozen" heart has a better chance of regaining a normal
  beat pattern than a fibrillating heart. This is why "defibrillating"
  equipment used by emergency medics works: the jolt of current supplied
  by the defibrillator unit is DC, which halts fibrillation and gives
  the heart a chance to recover.

There is a table with bodily effects at http://www.allaboutcircuits.com/vol_1/chpt_3/4.html
A: i think its depends upon the relative value of rms value of AC and absolute value of DC.
 sir if we touch a AC carrying  wire with one hand ,current will not flow in our body if we not connected to ground(potential difference zero= no current). but as we touch wire our body potential reach the value of wire potential, can our body support such a high potential? that may be cause of shock or death. pls correct me if m wrong.
A: DC is more dangerous in one respect in that it causes more electrolysis in the tissues than AC.
A: Realistically (statistically) speaking, AC is more dangerous that DC.  This comes from the fact that 120/240v AC is the voltage that we are most likely to encounter - which can kill us.  The DC voltage that we are most likely to encounter is 12 DC (in our vehicles), and it is very unlikely to kill us. 
For the case that we are just evaluating equivalent RMS AC vs DC, the AC would be more lethal, as explained in the other responses.  
