A moses basket or bassinet is specifically for babies from birth to about four months, and small enough to provide a cocoon that small babies find comforting.
They are generally designed to allow the resting baby to be moved to sleep to be close to its mother. Within the home they are often raised on a moses basket stand or other surface to reduce back strain when bending over to tend the baby. Moses baskets are also ideal for use when visiting friends or relatives when you may be staying over night.
Once babies are able to roll over by themselves, at three or four months of age, it is no longer safe for the baby to be left in a bassinet as they could tip themselves out, so they must use a baby cot instead.