Length
Choose the length you need. Bear in mind, you'll need to leave a little space for the wire at one end. If you're buying a light with a push switch, you'll need to leave a little space for that at the other as well.
Mount Type
3 choices. The photo's will help you out a lot with this option. The hospital brackets are easy to clean, so great for somewhere like over the hob.
Brass Munsen Rings
Brass Hospital Brackets
Copper Saddle Clips
Switch Type
No Switch - You supply an external switch or dimmer. The light will be supplied wired permanently in the 'on' position.
On/Off Push Switch - A 'clicky-clicky' on/off push switch poking out of one end. This won't do dimming, just on or off.
Touch Control Dimming - To switch the light on or off, tap it anywhere on it's surface. Tapping and holding will increase or decrease the brightness. It's very good.