Nitrogen-Vacancy (NV) centers in diamond have emerged as a leading platform for an ultrasensitive tool to study magnetic phenomena at the nanoscale. This is due to their atomic size and spin-sensitive fluorescence that enables sensitive transduction of magnetic signals into optical signals. Magnetometry schemes employing single NV center and sophisticated pulsed protocols have shown the significant potential of NV based optically detected magnetic resonance (ODMR). We have extended the functionality of NV-based MR detection by demonstrating broadband spectroscopy of both ferromagnetic and paramagnetic spins in magnetic field orientations both parallel and transverse to the NV axis. Our spin-relaxation based modality uses an easily accessible continuous wave protocol where the NV spins themselves are non-resonant, and is compatible with using ensembles of NVs in a high NV density diamond to enhance sensitivity. We have performed spectroscopy of ferromagnetic dynamics in a wide variety of materials and for different ferromagnetic excitations such as the uniform mode FMR, high wavevector spinwave modes and domain-state related excitations. We have also measured the hyperfine spectrum of paramagnetic P1 spins within diamond. In the case of ferromagnetic dynamics, NV spin lifetime measurements indicate that ferromagnetic damping processes cause an enhancement of the NV relaxation rate, thus changing its spin state and fluorescence, and resulting in the observed optical detection of ferromagnetic dynamics. For the case of paramagnetic resonance detection, we propose a two-phonon Raman spinrelaxation process leading to the observed signal. I will discuss our experimental observations and the evolving theory to explain our data. Our experiments should provide new approaches to studying magnetization- and spin-relaxation processes, and enable more widespread adoption of NV-diamond based spectroscopy for studies at the micro to nanoscale. This work is done in collaboration with groups of P. C. Hammel, G. D. Fuchs and F. Y. Yang.