They are abbreviations for the Latin exempli gratia (for example) and id est (that is). The choice of italic (to indicate Latin) or not is arbitrary, according to the publication; whichever is used, care should be taken that the same form is used consistently throughout the publication. The comma is not necessary and could be confusing coming immediately after a fullstop. I suggest that it is helpful to regard the full stop in abbreviations as standing for any missing letters (e.g. D. Smith instead of David Smith, but Dr Smith for Doctor Smith). I have always heard them pronounced as eee geee and I eee, as in normal letters of the alphabet. I hope this helps.