Depth of coverage (mapping depth)
How strong is a genome "covered" by sequenced fragments (short reads)?
Per-base coverage is the average number of times a base of a genome is sequenced. The coverage depth of a genome is calculated as the number of bases of all short reads that match a genome divided by the length of this genome. It is often expressed as 1X, 2X, 3X,... (1, 2, or, 3 times coverage).
Breadth of coverage (covered length)
How much of a genome is "covered" by short reads? Are there regions that are not covered, even not by a single read?
Breadth of coverage is the percentage of bases of a reference genome that are covered with a certain depth. For example: "90% of a genome is covered at 1X depth; and still 70% is covered at 5X depth."