This digital footprint will consist mostly out of a timestamp, an activity name and a case ID. This data is all captured and can be found in the Event Log, which is a record of everything that takes place in order to perform a task. Event Collection will thus bring all the event log data from every operational system in an organization together so it can be analyzed for process performance

Thank you Jain for your explanation. It seems they are the same thing but I still couldnt get the difference.

As @jain.saksh already explained: the event log is the data set where all digital footprints are stored. If you see it as a table, then a digital footprint is a row in the event log table, being a combination of Case ID, Activity Name and Timestamp (and optionally other attributes).

Okay, now I get it.