Service types are the different things you do for your clients that they are paying you for. They are used on Deals and Budgets and people track their time against them.
Go to Settings > Service types to set them up.
There will already be a list of default Service types in your account, but you can add new ones by clicking on Add service type.
You can delete Service types too, but be careful, people track time against them so you risk losing time entries if you delete a Service type that's in use within projects and budgets.