Each AEM template has a different set of components that can be defined to be available for use. This allows you to control what the authors will be allowed to use, and it makes it easier for them, because they will only see the relevant components, instead of the tons of components AEM offers.
When looking at a page, you can switch to something called the "design" mode (as opposed to the "edit" or "preview" modes where you spend most of your time authoring the page). This design mode allows to define the per-template specific settings. It is accessed through the yellow ruler icon on the very bottom of the sidekick.
When in design mode, click on the "Edit" button that is on the blue toolbar called "Design of par", there you'll be able to enable the components you want to be able to use.
When you'll be building components, keep in mind that the design mode and the corresponding design dialog of the components is a convenient way to define global per-template settings that you don't want to be required to be set specifically on each component instance.
Please have a look at this reference post:- http://help-forums.adobe.com/content/adobeforums/en/experience-manager-forum/adobe-experience-manager.topic.html/forum__njz2-hi_team_members.html
~kautuk