How are activity, taskand log a call different in salesforce?

So, an Activity is a general term Salesforce uses to describe a record that is either a Task or an Event. In Lightning Experience there's an Activity Timeline - this is essentially all Events or Tasks whose Due Date or End Date has passed. Log a Call just creates a Task, but it gives you a chance to use a different layout for creating Tasks right after calls. So maybe your New Task tab gives 8 different fields to populate, and Log a Call only has 3 fields to populate, making it easier to quickly create Tasks with only the necessary information after a phone call.