  Measurements of the rf penetration depth \lambda(T,H,\theta ) are used to
study the superconducting order parameter, vortex dynamics in the mixed state
and delineate critical fields in the borocarbide superconductor YNi_2B_2C. The
lower critical field has an anomalous T dependence, H_{c1}(T)=1.12[1-(T/T_c)]
kOe, which is however consistent with independent superfluid density
measurements at microwave frequencies. The vortex response is dominated by
viscous flux flow, indicative of extremely weak pinning, and is parametrized by
a field scale H_{c2,eff}. The angular dependence of the vortex contribution
\lambda(\theta) is in good agreement with the Coffey-Clem model. Structure is
seen in the depairing transition in the vicinity of the upper critical field,
with the existence of well-defined critical fields H_{c2a}, H_{c2b} and
H_{c2c}, with the vortex field scale H_{c2,eff} closest to H_{c2b}. Overall the
measurements indicate that YNi_2B_2C has a rich and unusual field dependence of
its transport parameters.
