Being new, I have general questions as I assemble my collection; Should I group used and unused stamps separately? I store my stamps in 8S and 5S sheets, and I have U.S. stamps, for example, in both size sheets. How do I know if a stamp is used/unused without a cancellation mark? I have stamps that seem to not be postage stamps, what might be the reason for their existence?
The traditional advice when mounting stamps on album leaves was to separate the unused from the used set, but you should do whatever you're comfortable with, especially if your collection isn't extensive. The terminology differs from country to country, but, in the UK, a stamp without cancellation but without gum would be termed "unused". Modern stamps are frequently not cancelled, even when they have been through the post. Where the stamp was not issued with gum, it would be called "unused, no gum, as issued".
Stamps were issued for a range of purposes, especially fiscal ones (to show payment of a tax). Others may be labels for propaganda or advertising purposes.
Again, it's a matter of taste and the value of the stamp. Modern collecting places a premium on mint stamps that haven't been hinged, so people tend to keep those in mounts, even if the stamps have almost no financial value. There's no reason not to hinge used or previously hinged mint stamps, unless the stamp has a higher value, but some people like the look of mounts, whereas others don't. If you're keeping stamps on stock-pages, you needn't worry about it!