Activities support use cases like "I need your help / permission" and "I need someone to remember to do this"

• available with all Apps
• access via the CLOCK icon at the top of the UI
• manage small chunks ( < 15 minutes) of work
• often completed in a single step
• always assigned to a single person
• can be delegated
• become part of the Chatter once completed
• integration only with the originating document needing the Activity
• Only audit trail is completion

https://www.odoo.com/documentation/15.0/applications/productivity/discuss/overview/plan_activities.html

Tasks support use cases like "We need to get this done" and "Lets schedule some time to complete this"

• available only with the Project App
• accessed via a Task Menu
• manage longer chunks ( > 15 minutes) of work
• often take multiple steps to complete
• assigned to nobody, or one person, or many people
• can be delegated 
• stay as Tasks and support Activities of their own
• integration with other Apps - Sales, Timesheets, Documents, Field Services, Human Resources
• Full audit trail - creation, delegation, stage changes, completion

https://www.odoo.com/slides/project-timesheets-21