Blender is a 3D modeling and animation program.
Blender is packed full of features, and rivals professional 3D modeling applications that cost thousands. It has far to many features to mention here, but as a brief overview:
The Blender user interface is flexible and has a fully configurable window layout with as many screen setups as you prefer.
For animation it supports skeleton deformation with forward and inverse kinematics, non-linear animation editor for mixing individual actions, automated walkcycles along paths, and vertex key framing for morphing
It has an extremely advanced physics and particle system, with particles that can be deflected by moving geometry and a fluid simulator with fully animated inflow, outflow, obstacle and fluid objects.