Often confused with e.g.: i.e. is used to explain, clarify or rephrase a statement; e.g. is used to list examples. i.e. differs from viz. in that what follows merely restates what was said in other words, rather than expanding upon what has already been said; and from e.g. in that completeness or near-completeness is suggested, rather than a small selection of examples.
American English prefers a comma after i.e.; in British English a comma does not follow i.e.
Opinion is mixed about whether the abbreviation should be italicized, or whether there should be a separating non-breaking space as in i. e., or whether this matters at all. However, the AMA manual of style recommends to forgo italic on terms long since naturalized into English and not to separate abbreviations (see "Abbreviation" on Wikipedia).