Classes of goods and services are an internationally agreed-upon system of categorizing businesses seeking trademark registration based on their offering.
The reason for devising this system is to evaluate two potentially similar brands on fair merits. If they operate in different industries, they are unlikely to confuse clients as to who the supplier of their goods/services is. Conversely, if two businesses operate in the same sector, less similarity is required between the two trademarks to find a likelihood of confusion.
There are 45 classes, with the first 34 classes covering physical products and the remaining 11 classes containing services categories. Each class breaks down into a list of goods and services, which you'll also have to define on your trademark application.
Choosing the correct goods and services is arguably one of the most challenging parts of the trademark application process. To give you an example of how this might look, if you are selling T-shirts, you'd choose Class 25, which contains clothing. However, if your business also sells leather vests, you might want to submit an application with two classes - Class 25 for clothing and Class 18 for leather products. The number and the exact choice of classes depend on what your business does and on some strategic decisions, such as budget (each additional class costs extra), expansion plans, what other similar trademarks already exist in the given class, etc.
You can read more about classes and find the complete list in our article Understanding goods & services selection.