You write e.g. when you want to give one or two samples, not a complete list. It means “for example.” A teacher might write “citrus fruits, e.g. oranges and lemons”: those two are samples, not every citrus fruit there is. It comes from Latin exemplī grātiā, “for the sake of an example.” English first wrote that as ex. gr., then shortened it again to e.g. You see it in textbooks, notices, and careful writing; people often put a comma after it. When we speak, we nearly always say “for example” instead of reading the letters out — saying “ee-jee” aloud sounds a bit stiff, though you will hear it in lectures. Though actually, there is also a thin noun use built from the abbreviation (“an e.g.” meaning an example), but learners almost never need that. What they do need: e.g. is not i.e. (which means “that is / I mean this, restated”), and it is not etc. (which means “and the rest of the list”).