  We introduce the Simba simulations, the next generation of the Mufasa
cosmological galaxy formation simulations run with Gizmo's meshless finite mass
hydrodynamics. Simba includes updates to Mufasa's sub-resolution star formation
and feedback prescriptions, and introduces black hole growth via the
torque-limited accretion model of Angl\'es-Alc\'azar et al. (2017) from cold
gas and Bondi accretion from hot gas, along with black hole feedback via
kinetic bipolar outflows and X-ray energy. Ejection velocities are taken to be
~10^3 km/s at high Eddington ratios, increasing to ~8000 km/s at Eddington
ratios below 2%, with a constant momentum input of 20L/c. Simba further
includes an on-the-fly dust production, growth, and destruction model. Our
Simba run with (100 Mpc/h)^3 and 1024^3 gas elements reproduces numerous
observables, including galaxy stellar mass functions at z=0-6, the stellar
mass--star formation rate main sequence, HI and H2 fractions, the
mass-metallicity relation at z=0 and z=2, star-forming galaxy sizes, hot gas
fractions in massive halos, and z=0 galaxy dust properties. However, Simba also
yields an insufficiently sharp truncation of the z=0 mass function, and
too-large sizes for low-mass quenched galaxies. We show that Simba's jet
feedback is primarily responsible for quenching massive galaxies.
