I will start today with some of the terms that I have good scans for, and then will continue to add to it as terms come to mind.
Air Mail Cover
Circular Date Stamp (CDS)
A comb perforation is one where the sheets of stamps are perforated either by a single stroke, or a few strokes of a perforating comb, in which the pins are all pre-spaced. This is in contrast to the more traditional line perforation in which the sheets are perforated one row at a time. You can recognize a comb perforation by the fact that there is always a perfect intersection of holes where rows and columns meet, as shown in the centre of the above block.
Cancellations for a long period between the 1860's and 1890's were often fashioned by local postmasters using corks that they would carve patterns into. These would then be dipped into the cancellation ink and then used as handstamps. Segmented geometric designs, such as those shown above are the most commonly seen, but occasionally some postmasters with superior carving skills got very creative indeed, producing highly sought after designs, such as the Waterbury Running Chicken cancel shown below:
On Canadian stamp booklets printed by the British American Bank Note Company, every 50th booklet was marked on the cover with a solid rectangular marking as a way of keeping track of how many booklets were printed. The above booklet cover shows an example of such a marking at the top centre, in the form of the solid red rectangle. These are highly sought after by specialists, due to their scarcity.
A cover refers to an entire envelope which has been sent through the postal system, complete with the original stamps used to pay the postage, called the franking. In the very earliest days of stamps, up to the 1850's, envelopes were not used. Instead the letters were folded, sealed with wax and then the outside was addressed and sent. These are also covers.
Cutting Guide Line
However, guide markings are usually placed in this case in the margins of the sheet so that the guillotine operator knows where to position the stamps, so as not to damage them of leave a margin around them that is too small. If the guillotine operator lines the markings up perfectly with the guillotine, the cutting action will split the guide line down the middle, and it will not be visible on the resulting pane. However, the markings and guillotine are sometimes not perfectly aligned, with the result that the cutting guideline is visible, as on the block shown above. These varieties are usually quite sought after by specialists.