Components give you more tools to interact with your contacts. You can use as many or as few of these as you need and then can be turned on and off as necessary. Turning off, or disabling modules that you aren't using removes them from the user interface, e.g. from the menu system, the advanced search page and so on. This simplifies the appearance of CiviCRM for your users. You can also set permissions so that only certain users can see certain components, for example so that only fundraisers can see financial information.
Turning off a component will not remove any data from CiviCRM - just hide it. When you turn it on again, the data will re-appear.
Each component is explained in detail in its own section so only a brief description is given here.
An online fundraising and donor management component which enables you to track and manage contributions to your organization. It also allows you to quickly and easily create customized web pages to accept one-off and recurring online donations. Part of CiviContribute is CiviPledge, which allows you to record and track pledges, send pledge acknowledgements and payment reminders, configure online contribution pages to support self-service pledging and pledge payments.
A robust mass-mailing component which allows you to engage your constituents with personalized email blasts and newsletters.
A flexible membership management component which includes convenient online signup and renewal.
Provides integrated online event registration and management for paid and free events. It allows you to quickly and easily create customized web pages for event registration, and then track participants.
Report templates that can be configured to provide information on the data in your CiviCRM installation.
Lets you link together events, mailings, activities, and contributions under one "umbrella" so that you can track the progress of all your efforts towards one programmatic goal or campaign.
Integrated case management component, including a dashboard with "my cases", automated case timelines and configurable audit reports.
Designed to be used by organizations that distribute funds to others, for example foundations, grant givers, etc.