I would like to know the pros and cons when a pc is on or off when not in use. Also, are there any credible references I can use to help go more into detail regarding the question?
Here are some non-contentious advantages of switching the PC off when not in use:
+ Save power (see http://energy.gov/energysaver/articles/energy-efficient-computer-use. For general reference, most desktop PCs use about 15W power in sleep mode, although some modern PCs use less than 1W).
+ Reduce exposure to damage (hacking, malware, bugs, power surges...)
+ PCs running for a long time can suffer from memory leaks, and potentially from certain bugs which are more likely to affect the machine after some time.
Likewise, some advantages of leaving it on are straightforward:
+ Convenient, no need to wait for bootup
+ The system is available for background activities, such as downloading updates, backing up data etc, at any time.
The more contentious areas is the possible effect effect of switching off/on on life of components, and particularly the HDD. The theory is that the electrical and/or mechanical jolt a component receives when turned on will shorten its life, and that the expansion and contraction of the components as they heat up and cool down will affect their lifespan too.
I've read about this widely over the years, and have never found any studies or clear evidence one way or the other. Hard discs are regularly "put to sleep" by the operating system when not in use, and other components (e.g. fans) are also put under variable load depending on what the computer is doing, so it would seem that the designers and manufacturers of the hardware and software are not worried about the effect such designs have on the life span of the components. On the other hand, temperature affects both electronic and mechanical components adversely (there is information on that in Wikipedia, but unfortunately I do not have the reference to hand), so one could argue that leaving the computer off for some of the time will extend its life...
My suspicion is that issue has become such a hotly debated one because in the old days (80's, early 90's) it was necessary to convince the doubters that leaving a computer switched on was not such a bad idea (it saved a lot of time, and there was no discernible effect on the lifespan of the devices). Over time, this idea has turned into the dogma that it is actually *better* for the devices, but as I say, I've never found anything other that people's personal opinions to corroborate that.
You will find an intelligent article on the subject at : http://www.computerhope.com/issues/ch000390.htm
For what it's worth, my own practice is to switch the computer off when I do not intend to use it for an extended period of time (e.g. in the evening, or if I'm leaving the house). If I'm just taking a short-ish break (e.g. lunch), I leave it on. That said, that's MY preference, and you were quite right to ask for references!
Leaving your machine on will result in less wear and tear on your hard drive.
Computers high-end desktop use more energy then lower end pc so shutting your computer will prevent unnecessary use of power so saving you money on your electric bill.