Stamp designs used to be printed one design per sheet, with all stamps in a sheet being exactly the same. Starting in the 1950's many issues, usually commemoratives, consisted of three of four designs, which would all be printed in rotation on the same sheet. These are called se-tenant designs. They are not uncommon in mint condition, but properly used se-tenants on cover, used in the proper period of time (i.e. when on sale at the post office) are some of the most challenging items in modern philately.
The above scan shows the 2000 "Stampin the Future" issue, which consisted of four se-tenant designs.
A souvenir sheet is a small sheet consisting usually of fewer than 10 stamps, which has a very large decorative border. These sheets were first issued just before World War I, and have continued to increase in popularity over the years. Originally the sheets issued in the 1920's and 1930's were generally only available at philatelic exhibitions and you could only buy one sheet with one exhibition ticket. As a result the issue quantities of early souvenir sheets were quite low and these are worth a lot of money today. By the 1960's most souvenir sheets are only worth a small premium over the basic stamps contained in them. Once again, mint sheets are not uncommon, but used ones on cover, used when on sale are rare, and highly desirable.
Special Delivery Cover
A surface cover is one that traveled exclusively by land and sea. Surface was the standard method of transmission until airmail became popular and affordable in the 1950's. However, airmail did not become the standard method of transmission until the 1970's, with surface still being offered as a less expensive option. Today, surface is generally not available in many countries for lettermail, though it is often still offered for parcels.
Envelopes sent by surface were generally not marked, though sometimes "by sea" or "sea mail" will appear on the envelope. Most of the time though, the only way to identify a surface cover, particularly for modern covers, is to know the postage rates. Quite often, pre-printed airmail envelopes were used for surface mail, such as in the case of the above cover to the Canary Islands. This can mislead the unwary collector into thinking that the cover went by air when it did not.