The Call Log is basically a log of every call received by YAC.NET.
The calls are added to the end of the log, and are stored with the date and time that they were received. The date and time are those of the computer, and are not gathered from the Caller ID information transmitted by the phone company.
The number stored is the same as that transmitted over the phone line. The name stored is either the one transmitted in the Caller ID information, or the friendly name entered for that number in the Phone Book.
You can print the Call Log, delete individual entries (select and hit del, or right click and choose "Delete"), or clear the entire log.
If you want to add a number in the call log to your Phone Book, right click on it and choose "Add to Phone Book...". The phone book dialog will open and allow you to edit the entry and add a friendly name.