Overview
What are International Trademark Classes? Well let’s keep things simple, There are 45 Classes in total (As of March 2020) listed at The World Intellectual Property Organization (WIPO)’s website. There are 34 classes for Goods and 11 classes for Services. Those classes define the “relationship” between the Trademark and the Goods and Services that the trademark claims to provide.
When the application for trademark is submitted, say to upsto.gov those classes are essential in order to provide protection and provide clarity to what the trademark is all about 
Example time
Let’s look at two companies that are registered under the same trademark “APPLE”. Company A builds computers and other electronic hardware while company B sells Tobacco. Now to make things simple the computer company used IC 9 (International Class 9) to describe the Goods and Services while the Tobacco company used IC 34 (International Class 34). Now, in theory, both companies can conduct business but it’s probably not a good practice to file your trademark if one already exists in the similar field of business. This might be a recipe for trouble.
Full list of International Trademark Classes
You can find the updated list on The World Intellectual Property Organization (WIPO)’s website www.wipo.int
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