Few things that makes a lens stand out are:

1. Aperture
2. Sharpness
3. Focal length
4. Build quality
5. Other features: (IS/VR, FTM, Distance Meter etc)

Aperture Some lens have fixed aperture throughout the entire focal length and some have variable. Most lenses with fixed focal lengths are generally better and considered top notch. Some lenses have fast aperture (ie. f/2.8) but the sharpness you get on that aperture is close to unusable, so make sure your lens has descent sharpness on entire focal length using the widest aperture. The wider the aperture is, more light can enter the lens and this gives you the ability to do better in low light conditions.

Sharpness Most lenses produce softest image on the fastest aperture, and sharpness generally increases when aperture is stopped down a stop or two. Top quality lenses doesn't compromise image sharpness even when using the widest aperture.

Focal length: This is something you'll need to decide. There are zoom lenses and fixed/prime lenses. Primes have better sharpness in general. You can go as wide as 8mm and as long as 1200mm (of-course not in the same lens) these days. General purpose lens are considered to have 15-24mm on the wide-end and 70-135 on the tele-end. Super zooms (ie. lenses having 5X+ zoom) generally do not produce high quality images, unless you want an all-in-one solution, do not go for a super zoom.

Build quality Good lenses have metal mounts, weights more and some are weather proof. Pro series lenses have better build quality than non-pro lenses.

Other features: Stabilization: Its called different names for different manufacturers but more or less works the same. It helps you to take hand-held picture of non-moving subjects better and is a nice to have feature. Please note that, this feature often add a huge money on the lens and drains more power from your battery. Also IS doesn't keep your subject from moving, it can not freeze motion. FTM: this gives you the ability to fine-tune focus without having to switch to MF. Distance Meter: Gives you subject-to-camera distance. Non Rotating Front Element: This means the front element does not rotate when you change focus. This gives you ability to use CPL filters better. Perfocal: These lenses do not change focus when zoomed in out out.

Actually there are a lot more, but I believe once you start, you'll learn them all. I highly recommend www.the-digital-picture.com for lens reviews. They provide ISO chart crops to compare sharpness between two selected lenses at different focal length and aperture. This feature is quite useful to check what the lens is capable of.