The Earth is always being drawn toward the sun by gravity. With no gravity, the Earth would simply coast off into space, since you need force to change something's direction when it's moving. The Earth doesn't fall into the sun simply because it's moving too fast, and the sun's gravity isn't strong enough to make the Earth come any closer, only keep it at the same distance. This means the Earth goes around the sun, keeping roughly the same distance from the sun at all times - in other words, its orbit.