Most of the built-in bill templates are set up to show both the category and the details/notes of time entries and expense items. The field that most bill templates use is called [description], however, you can change this field to be just the category or just the notes, if you prefer.
To do this, when customizing your template, click on the field in the template that is labeled "[description]" and click the ">" which will appear once it is selected.
This will pop-out a little window with a number of settings you can change for this field - the one you want is the "data binding" option.
Click this drop-down box to change the information which is merged into this field in your template - for example, you might want to change it to [notes] so that only the details/notes you've entered appear. Alternatively, you might want to change it to [category] so that only the category appears.
Once you've made your changes, you can click on the Print Preview tab to see a sample of how they will look, and when you're done, click the Save button to save your changes.
You can use this method to change other fields in your templates as well; you're not limited to just the description field. You can find a full list of available fields in this article.