When levied as punishment for an infraction of some type committed by a military servicemember, an Article 15 applies to minor offenses. Those can generally be distinguished quite readily from more serious misconduct that is termed criminal in nature and, in the military, is typically dealt with in a court-martial proceeding.
That, what is immediately clear about an Article 15 (referred to as such in the Army and Air Force, with other branches having similar punishment under different names) is that it is not a court-martial, which deals with more grave matters. In a general court-martial, for example, a military member is in a trial setting and faces one or more charges that, if dealt with in a civilian court, would be viewed as felonies, such as murder, robbery, rape or assault.
An Article 15, conversely, is deemed non-judicial punishment, which means that punishment -- if forthcoming at all -- is doled out by a commanding officer or officer in charge rather than by a military judge after a trial has concluded.
Here's a caveat, though: As noted by a media overview discussing Article 15, its imposition "does not, in all cases, preclude a subsequent court-martial for the same offense."
Typically, though, such is not the case, given the stated emphasis on minor offenses where an Article 15 is involved. What constitutes "minor," notes the above-cited article, "is within the sound discretion of the commanding officer."
Notwithstanding that an Article 15 is seldom deemed a career killer, it can still bring about adverse consequences for an alleged offender, who might reasonably want to confer with an experienced military defense attorney to evaluate options.