I'm sorry if this answer is too general for you, but it's not clear to me what you're actually looking for.
More expensive lenses, and primes, have features like:
- sharper images
- better build quality
- wider apertures (helps in low-light situations, but often used for artistic reasons)
and these differences are discussed ad nauseam throughout the Internet so I won't go into detail here.
But when deciding if you want to spend more money on a lens, the best first question to ask yourself is: what is it that I want to do that I can't do with my current lenses? Once you identify that, it should be easier to decide what lens you need to get. You'll also be able to ask a more specific question, or find one that someone else has already asked and had answered. Remember also there really isn't a do-everything lens, so if the two lenses you already have aren't enough for you, you're probably going to start getting into more-specialized lenses.
If you really just want to know what are the differences between two lenses, there are plenty of lens review sites out there that will help you (see Where can I find reviews of lenses?).
So I would really recommend trying to understand better what your lenses are preventing you from doing. You might need to dig a little deeper than just, "my portrait shots aren't good enough," or "that animal isn't big enough," because there can be many reasons why your pictures aren't turning out the way you want, and getting a more expensive lens won't solve them.
Finally, two more non-lens bits of advice:
- for portraits: you'll want to learn how to use light, which is probably more important than the lens you use.
- for pictures of distant objects: you'll either want a good tripod or a lens with VR (vibration reduction, a Nikon phrase; also called IS (Image Stabilization) and many other terms) in order to get sharp images, and those get much more important the longer your lens (higher number of mm) gets. Also the best zoom method is your feet: get closer to the object, so that it "fills the frame," and you'll get a better picture.