In Salesforce, we assign tasks and events to certain users based on the requirement and it might find difficult to remember details about each and every task or event which has to be completed on time. So, in Salesforce CRM we have reminders which displays tasks and events by a popup window showing the details, subject, time etc whenever user opens salesforce account.

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In another way salesforce automated reminders can be sent via email automatically whenever task or event is created, this is done using workflow rules.

In the Activity settings make sure to enable Activity reminders.

Reminder through popup window:

A popup window with events and tasks are displayed whenever a task reminder is triggered.

Process:

Open a custom object which is already saved and in the activities section click on new task or event.

In the new task, assign the task to user.

In the subject, click on the lookup field select already predefined subjects like call, email, send letter etc or give the subject name according to the requirement.

Mention the priority, status, name of the contact and relation to the task.

In the reminder section, provide the date and time so that if the given task hasn't been completed by that date and time, it will remind the assigned user with due date and time.

When the task or event is triggered, it displays popup window containing subject, type and due in.

It shows task completion date and time with the object that is related to and also the due time.

We can select one task or multiple tasks to dismiss the tasks upon completion and also we can dismiss all the tasks at a time by selecting dismiss all option. 

In the snooze option, we can select time and make the reminder to appear again when the given is completed. This is similar to the snooze option available in mobile alarms.

Reminder through Email:

Reminders can be sent for tasks or events through Emails to the assigned users. This can be done by writing Workflow rules.

Workflow rules are useful to automate the business process. For example, automatically send an email to assigned user whenever the given due date is less than 2 or 3 days to complete the assigned task or event.

Write a workflow rule such that when a task is overdue by date and time it should send an email reminder to the task assigned user.