Understanding layers in Eagle schematic window?
In the EAGLE reference manual of version 5, the description of layer 98 is more clear:

98 Guide:    Guiding lines for symbol alignment


So, use this layer for all lines and forms you need to align the symbols in your schematic - and not for e.g. guidelines for assembly of the PCB.

The Info layer can be used as explained by Olin Lathrop: for any additional comments and information. I've also used that to describe what is attached
 to a connector, e.g. a motor or LED.