Excise / Revenue - varies by country and era - these stamps are used to collect some tax. They have been used to indicate taxes on things from alcohol to potatoes, and in some countries are used by normal people to pay tax for a service.

Postage Due - not common today, but once used to indicate all or part of the postage which was to be collected from the person getting the mail - not sending it!

Regular - stamps of many values used for many years, not usually very pretty.

Semi-Postal - commemorative stamps with an extra charge for some special charity purpose. You can tell by seeing the value as number + number. Not common and not all countries use them.

Special Delivery - not common today, but once used for a letter requiring delivery outside normal mail times. Today there is EMS service.