So I am sure you have noticed how expensive camera lenses can be. And more than likely, the cheaper ones are going to be more appealing right off the bat. I remember a time when all I wanted to get was one of those $200 dollar zoom lenses. Which at that point in my life seemed like A LOT of money to save up for a lens. Thankfully I never invested in one though. And without getting too technical, here  are a couple basic reasons why:

The cheaper lenses are cheaper for a reason. The quality of the glass they are made with is not near as high,  and thus your pictures are not going to be as good.
Also, the cheaper lenses have a higher aperture to start out with, which means you're letting less light (also think--grainier pictures) in your camera and you won't get as shallow a depth of field. And to top that off, the more you zoom in with these kit lenses, the greater the aperture gets.  A typical kit lens is an 18mm-55mm f/3.5-5.6. That 3.5-5.6 is the aperture range, so while 3.5 might be nice for 18mm, at your really wide angle, by the time you're at 55mm, trying to take a nice portrait, the lens is at 5.6. So avoid these lenses when you're buying a new one. Just skip 'em. And compensate until you have the money to invest in better lenses.
They cost more money, but if you're serious about photography--They are well worth it. And honestly necessary.

I could go on about what makes up a good lens, but an even greater resource I would recommend is Ken Rockwell's reviews. He has reviewed just about every lens and camera body known to man--At least as far as Canon and Nikon are considered. He gets very in depth and can let you know pretty much anything and everything you need to know about a lens. So if you're on the market for a new lens, or you just want to learn a bit more, check his site out. The best way to find a specific lens review is to just google it--Type the lens name, along with Ken Rockwell, and it'll pull right up.

Here is a before and after picture to give you a visual.
The first picture I took with my 18-55mm kit lens, way back when. And at the time I was really proud of this shot. However,  I really struggled to get it. And as you can tell, there is motion blur, it's not a super sharp focus, and the picture is grainy.
The second shot is with the same camera that I started out with, but I was shooting with a 50mm prime lens. (See why you should start with a 50mm) The image came out brighter, cleaner, sharper, and a lot less noisy.