The manual should contain whatever the owner of the device needs to know, including but not limited to:
- Unpacking: how to unpack it
- Operation: how to operate it
- Integration: how to use it in conjunction with other devices, if appropriate
- Care: how to care for it
- Supplies: where to get any supplies that the device requires
- Troubleshooting: what to do when it doesn't seem to work
- Service: who to contact if it needs repair
- Safety: any necessary safety information
- Recycling and Disposal: how to get rid of it responsibly
- Warrantee: statement of warrantee and any disclaimers or other legalese
- Index: a thorough index
If using the device is complicated and/or if the users aren't expected to already be knowledgeable, it may be a good idea to include some examples.
I'm not aware of any general manual-writing standards, but I'm sure that some exist for specific markets. For example, one can easily imagine that you'd have to supply a manual meeting certain criteria if you plan to sell to the military, and certain countries might require that the manual be provided in the official language of that country.
For ideas, take a look at the manuals that came with your car, blender, stereo, DVR, printer, microwave oven, and any other devices you have around your house.