The other kind makes stuff up as they go along. They have different names but one of the most common is "Pantsers," for "seat of the pants" writers.
I am the second type, though I find that name rather obnoxious. Why does one group get a name that sounds very writer-ly and the other gets a name reminiscent of grade-school humiliations? I don't know; "Improvisers" sounds nicer to me.
I almost wonder if these names are a way to imply that there's only one right way to go about writing, and that everyone should eventually graduate from that amateur-ish title and become a Plotter. I don't know....I'd be very interested to see statistics as to the percentage of Plotters that finish writing their books, versus Improvisers.