Mac events and reminders are those that are managed and synced by OS X. For example your iCloud or Exchange events and tasks are available system-wide in the Apple Calendar app and Reminders apps - and also within Desktop Informant. With these events and reminders, OS X handles the alarms, the synchronization, and all aspects of the item even though its visible and editable within Desktop Informant.
Informant events/tasks are those that Informant syncs and manages directly. These are stored in our database and are not available to the rest of the system. Informant handles the alarms, synchronization, and all aspects of these items.
Pros/Cons
The advantages to a Mac event/reminder is that its available system-wide, works with some accounts like Exchange that Informant doesn't natively support, and sync even if Informant isn't open. The negatives are that they are a bit slower and do not support all of Informant's features. You are also limited in that you cannot edit attendees or send meeting invitations from Informant.
The advantages of Informant events/tasks are that you have many more options for the events/tasks such as tags, meeting invitations, subtasks, checklists, and more.