Oracle Analytic Functions - Difference between ORDER BY and PARTITION BY

ORDER BY - brings up the running total/sum by certain repeating column
PARTITION BY - brings up the GROUP total by certain repeating column

Consider following simple table.

This projects example contains 2 projects - ABC and CAB
Each project contains 2 tasks and its cost respectively.

By using same Analytic functions with two different window, can get "running totals" and "group totals"

select project_id, project_name, task_no, cost
,sum(cost) over(order by project_id,task_no) running_total_by_project_task
,sum(cost) over(order by project_id) running_total_by_project
,sum(cost) over(partition by project_id) total_cost_by_project
,sum(cost) over(partition by task_no) total_cost_by_task
from temp_project_details;

Output below..