There is no way I can describe how to draw fur in one post, so I'll lay down the basics first. Look at any animal with fur and you'll notice that the fur goes in a certain direction. The body is obvious, it starts at the head and goes down the back, down each limb, and down the tail. The head is much more complex: fur goes from the base of the ears to the tips; around the eyes starting at the tear ducts; sideways along the cheeks; and from the nose to the forehead. It is the intersections of these areas that can be difficult to determine fur flow. Look closely at any cat or dog between the nose and one eye. There is a point where the fur reverses direction (see image).
Now, that being said, from some angles and in some poses, parts of the fur may appear to be going the wrong way. This is where the fur is lifted or twisted by the muscles underneath or because of the perspective. Take, for example, my koala. In the dark areas on the right arm, the fur has been lifted up and casts a dark shadow. It appears to flow from the base UP to meet with the fur above it. In my cat drawing, the chest fur goes in all sorts of directions. It is essential to incorporate these variations or the drawing will not look real.
If the animal has a short coat, like a deer or horse, there will most likely be none of this. Short coats don't fluff up - they lie flat against the body (but you'll probably still end up with that wierdness around the eyes).
Even if you are working directly from photographs, it helps to have familiarity with the pattern of fur of the animal you are drawing to ensure you get it right.