Ground covers are low-lying plants (and often vigorous spreaders), usually chosen with aesthetic considerations in mind and requiring minimal maintenance. If they are projected to cover large expanses of ground on the landscape, the initial cost will be much greater than for sowing grass seed, but ground covers may save you money in the long run, as expenses such as tune-ups for lawn mowers will be eliminated.
"Ground covers" and "cover crops" are different groups of plants, despite some overlap. Cover crops are sometimes used as "living mulches," in which function they serve a role, not unlike ground covers. But cover crops are often tilled into the soil not long after being planted, simply to supply the soil with amendments, whereas ground covers may inhabit a stretch of land for many years.
Ground covers can be saviors for homeowners who are stuck with having to mow steep slopes (or who are simply unable to get grass to grow in such areas). If you are plagued by such a hillside and it is in a sunny area, my article about 'Blue Rug' juniper ground covers may be the answer to your problems.