High-resolution transmission electron microscopy (HRTEM) and cross-sectional transmission electron microscopy (XTEM) are used to characterize common defects in wurtzite GaN nanowires grown via a vapor-liquid-solid (VLS) mechanism. HRTEM shows that these nanowires contain numerous (001) stacking defects interspersed with small cubic GaN regions. Using XTEM, bicrystalline nanowires are discovered with twofold rotational twin axes along their growth directions, and are found to grow along high-index directions or vicinal to low-index planes. We propose a defect-mediated VLS growth model to qualitatively account for the prevalence of these extended defects, and discuss the implications of these defects for nanowire growth kinetics and device behavior.