On the default timer you can exclude the applications you won't be part.
The default timer sends an email with the error logs, then occurred more than X times, and, the number of times of each error occurred from the last X days. You can change these X values on Service Center in site properties.
On the custom timer, you can define the specific applications you want to be part of this.
The custom timer sends an email with all error logs that occurred from the last X hours. Also, you can change this value site properties or on the input is the screen.
A search menu is also available to search error logs by application name.