Uptime is a measurement of how long your computer has been running without having been rebooted or turned off. Your uptime shouldn't be longer than about 16 hours, maybe 18. You have to sleep sometime.
Unfortunately, many people never turn off their computers. Energy use is at an all-time high and is only increasing. While I'm not a dyed-in-the-wool environmentalist, I am opposed to needlessly wasting energy when doing so is utterly fruitless.
Some people need their computers on all the time, and there can be legitimate reasons for this. If you need to compile a large program overnight, go ahead. Rendering a huge 3D image? No problem. Need to encode a bunch of video files and want to run it overnight? Go for it.
I recently purchased a Kill-A-Watt electric meter, which is a device you can use to measure how much energy an electrical appliance is using. I used my kill-a-watt around both my and my parent's house (my Dad was really interested too), and took some readings. Some were surprising, others were not. It's worth noting that large appliances (TVs, refrigerators, computers) often have a spike in power usage immediately when they are turned on, but usage quickly drops once the startup time has passed.