“The first dimension represents candidates’ overall ideological positioning from left to right, relative to other Republicans. (In this context, “left” means moderate or center-right, while “right” means quite conservative.)

The second dimension, which runs from the top to bottom of the chart, is what I call the insider/outsider or the establishment/insurgent axis. It has do to, in essence, with whether the candidate is running a grass-roots campaign that argues against entrenched interests or is trying to use those interests (like access to money and favorable media coverage and endorsements from influential party officials) to their advantage.”

Good and deep analysis about G.O.P. Candidates by Nate Silver.