Chainrings are sized with two factors, the number of teeth and BCD. BCD stands for Bolt Circle diameter, PCD is Pitch Circle Diameter, it means exactly the same thing but in engineering terms PCD is more correct. Your chainring is bolted to the cranks by commonly five or four bolts PCD is the diameter of the circle these bold describe. With a four bolt MTB system it is pretty easy to measure the diameter, centre to centre on opposite bolts. Five bolt system presents you with a quandary as there are no opposite bolts. The mathematical way over this is the distance in millimetres between two adjacent bolts multiplied by 1.7 and Robert is your Mothers brother.
There are four or five common PCD's across the biking world and quite a few less common ones. Gods Fourth Hand, Sheldon Brown (citation needed) has an extensive list of common and obscure PCDs.
Things to trip you up. 110PCD. In the ongoing corporate bicker twixt Campagnolo and Shimano common sense went by the by. The PCD standard for compact chainsets was agreed as 110PCD, BUT Campagnolos' chainrings have one bolt hole a few millimetres out of line so Shimano compatible 110 rings will not fit. You will also find that the early Campagnolo compacts where one chainring bolt screws into the crank are a different thickness to modern ones. TA manufacture appropriate rings for all of these variants.
Campag are not alone in this PCD variations. Shimano's XTR chainsets have two standards depend on vintage. Usually a triple chainsets' middle and outer ring are the same PCD, bolting to either side of the chainset spider with the inner or granny ring on a separate PCD. Some XTR have three PCDs on one spider and others two where the middle bolts through the spider into the outer. Shimano's replacement rings are pricey to say the least and until recently TA did compatible rings at a better price. This will change this Autumn with the TA Sismic rings due for a significant price hike.
Brain folding yet? Mine to. Off to have lunch and build a bike.