The classes of Goods and Services define the area of business in which your trademark will be registered.
There are 45 main categories of the classes, with several thousands of sub-classes. Classes 1 to 34 are for Goods and 35 to 45 for services.
When you define the classes you should think big and into the future. Analyze your products or services and check where you want to go for the next 3 to 5 years and add classes where you think they might fit into your business.
Classes are also very important for the evaluation and protection of your brand. The broader the classes are defined, the higher the value and the better the protection, which is a logical fact.
But you need to consider, that you are using classes only that really fit into your company or product. If the trademark offices identifies that you are using classes that are not related to your business, product or service, the trademark could be withdrawn.