What does the “N rating”, or grade, of the neodymium magnets mean?

The grade, or "N rating" of the magnet refers to the Maximum Energy Product of the material that the magnet is made from.  It refers to the maximum strength that the material can be magnetized to.  The grade of neodymium magnets is generally measured in units millions of Gauss Oersted (MGOe).  A magnet of grade N42 has a Maximum Energy Product of 42 MGOe.  Generally speaking, the higher the grade, the stronger the magnet.

Do I have to worry about temperature with neodymium magnets?

Yes.  Neodymium Iron Boron magnets are sensitive to heat.  If a magnet heated above its maximum operating temperature (176°F (80°C) for standard N grades) the magnet will permanently lose a fraction of its magnetic strength.  If they are heated above their Curie temperature (590°F (310°C) for standard N grades), they will lose all of their magnetic properties.  Different grades of neodymium different maximum operating and Curie temperatures.

Will magnets harm my electronics?

Maybe…The strong magnetic fields of these magnets can damage certain magnetic media such as floppy disks, credit cards, magnetic I.D. cards, cassette tapes, video tapes or other such devices.  They can also damage televisions, VCRs, computer monitors and other CRT displays. Never place neodymium magnets near any of these appliances.  As for other electronics such as cell phones, iPods, flash drives, calculators and similar devices that do not contain magnetic storage media, probably not, but it is best to err on the safe side and try to avoid close contact between neo magnets and electronics.