A brown dwarf would be an object roughly the composition of Jupiter (probably liquid hydrogen), except a bit larger, but not large enough for the pressure to cause nuclear fusion at its core. They don't do anything except take up space and have mass.
If one appeared between Earth and Venus, the orbits of the planets (all of the planets, probably) would be significantly disrupted. Some planets would be flung out into interstellar space, some would be set on a collision course with either the brown dwarf or the Sun or each other, and some planets might somehow find a stable orbit, but it's incredibly rare to find a stable orbit around (but not including) a binary star system. The brown dwarf otherwise doesn't give off any heat or radiation, so it would be just like a really big Jupiter.
Whatever the situation, Earth as we know it would be gone.
