Ideally event logs are stored in the standard format for process mining XES (eXtensible Event Stream). However, the native format is seldom and event log. Often Comma-Separated Values (CSV) files are used as an intermediate format. The rows in a CSV file correspond to events and the columns to attributes of events. There should be columns for the case identifier, the activity name and the timestamp of an event, but there may be many more attributes.
ProM and most other process mining tools can convert a CSV file into an event log by assigning columns to process mining concepts.
The presentation What kind of data does process mining require? provides a few examples.