The Reminder Web Part allows users to configure options for sending out email reminder alerts.
The Reminder Service sends out the emails according to how you have configured the Reminder Web Parts.

Both should be installed on the SharePoint Web Server using the supplied installation package which you can download here

Note - Reminder contains both a Web Part and some server side software that runs as a service - for this reason you must use the supplied Installer as only the web part can be installed using a SharePoint WSP file.

To ensure that the Installer can complete correctly it should be run from a user account that is at a minimum a member of the Local Administrators group and has administration permissions for SharePoint.

If you have a separate SQL Server database then the user account you use must be a domain account that also has sufficient permissions on your database to perform SharePoint administration tasks (STSADM).

If you are using a single front end web server (either with MSDE or SQL Server databases on the same or separate servers) then both components should be installed on this front end web server.

If you are using multiple front end web servers then the Reminder Service & Configuration Tool should be installed on only one front end web server whilst the Web Part will be automatically deployed to all Web Front End (WFE) servers

If you are using SharePoint Server Subscription Edition or you have unchecked the option to automatically install the web part then you will have to manually install the web part.

Once you have installed the Reminder Service you may have to perform some additional configuration as detailed here.