The word "woodstain" means different things to different people. A quick explanation:
Wood dye: is mainly colour. Some have a low level of binder in them. They should be use only on bare wood, so they can soak in, and must be topcoated with a stronger, normally clear finish. Some can be combined with a compatible varnish to make a coloured varnish and they can also be combined with woodstains. This helps enormously when you are trying to match in new wood.
Coloured varnish: a colour combined with lots of binder or resin. A varnish sits on the surface of wood, rather than sinking in and is quite high-build offering a protective barrier of see-through colour.