The nearby low-mass stars are the best candidate hosts for searching for transiting exoplanets to enable future characterization of their atmospheres. Unfortunately, our understanding of exoplanets is most often limited by our ability to characterize the host star. In this talk, I will focus on this stellar characterization problem, and how the MEarth transit survey has enabled us to better understand these stars. MEarth consists of 2 arrays of 8 telescopes each, one located at Mt. Hopkins, Arizona, and the other at Cerro Tololo, Chile. First, I used data from the Northern array to measure precise distances to these stars. With these distances we better characterized the MEarth M dwarfs and selected a volume-limited sample from which to search for planets. Second, I calibrated the MEarth photometric system using observations of Landolt standard fields. Combined with our trigonometric distance measurements and spectroscopic metallicity estimates, I created a color-magnitude-metallicity relation for the mid-to-late M dwarfs capable of reproducing spectral metallicities with 0.1 dex precision. With these metallicities, we plan to measure any potential planet-metallicity correlation at the low-mass end of the stellar sequence once future missions uncover the planets orbiting these stars. My thesis has focused on characterizing the nearby M dwarfs, whose size and proximity make them the ideal planetary hosts for current and future exoplanet studies.