Havok (not "
Havoc") is the
codename for Alex Summers, mutant younger
brother of the better-known Scott Summers (
Cyclops) of
X-Men fame.
Havok has the power to absorb ambient cosmic energy and convert it into plasma blasts which he can direct. Technically (according to the Official Handbook of the Marvel Universe) he generates intense directed blasts of heat, which instantly ionize the air it passes through and converts it into plasma. The extreme heat of the plasma causes whatever it strikes to explode, which is why he's reluctant to ever use it on people (except his brother Scott, who's immune to his power, as Alex is to Scott's).
Currently in the Marvel Universe, Havok is tied up in an alternate universe where he leads a version of the X-Men and sometimes goes by the moniker "Mutant X". It's generally understood that this particular comic book is crap, although that's due to the ineptitude of its writer rather than anything fundamentally wrong with its premise.