It can actually get a little more complicated though... a lot depends on the community or the specific religious group involved, and in academic texts the two terms will usually be discussed (with the specific meaning given to each by the author) in an appendix or forward since the two terms do get used differently by different pagan groups.
Basically, Aleister Crowley used "with a k" (as a way to set himself apart from stage craft) and so groups/individuals that draw from that history tend to use "with a k" while groups/individuals that draw from other histories use "with a c". There's a nice wiki page here
that explains how/why Crowley coined the term and how it is used by different groups now.
But in the end... both "magic with a c" and "magick with a k" can be used to mean the same thing. It's always a good idea to ask the individual author if it seems like they are distinguishing between the two.ETA- I realized I was tossing around names that might have no meaning for non-pagans. Mea culpa! Crowley was a rather flamboyant and controversial founder of a specific type of paganism. His wiki page is here. He is a very important figure to some pagan traditions and completely uninvolved in other pagan traditions