Really, the biggest difference between laptops is the brand. Typically, more expensive laptops are better built laptops.
Dells tend to break after a year. HP's, 6 months if you do anything other than leave them in one spot on your desk, a year otherwise.
Lenovo's are basically rebranded IBMs, and are typically decently constructed. I've had good luck with Fujitsu as well, but only for their tablet notebooks (higher end).
Mac's are higher priced, and higher end, but are also built well.
Once you've figured out what brand you are buying, then the rest should be easier.
You can go with a desktop type processor (AMD Phenom/Intel I5) or a more mobile oriented processor (AMD Turion/Intel Atom), with the tradeoff being that typically higher power CPUs make your battery life short. On the higher power processor side, you can usually shop around for a discrete graphics card model, which would allow you some limited videogame playing.
From there, the options are usually HDD speeds and sizes (7200 RPM being fast, but power hungry, 5400 RPM being slow, but good battery life, and SSD being fast, good battery life, but small in space), screen size ( > size == > bulk/weight), and RAM/memory (less practicality for more RAM, and laptop RAM is usually much higher priced per GB).
So it depends on what you want to do.
If you intend to use it mostly on travel, pick one with a mobile style processor, a moderately sized SSD (maybe 32GB-64GB, with Linux, 128GB+ with Windows 7), and a smaller screen size/lighter weight.
If you intend to use it mostly as a desktop replacement, pick one with a good processor (Phenom/I5/I7), a nice sized 7200 RPM HDD (300+GB), a discrete graphics card, and a large screen.
And of course, one that fits your budget (which you should probably increase).
I hope that helps a little bit in explaining the options.