This is an all encompassing name for any rose that doesn’t fit in the other categories. The earliest ones were made from crossing various species roses with Hybrid Teas and Floribundas early in the 20th century. Since then their background has become very much more complex.
There is no common denominator: the flower size ranges from small to very large, petal number from 5 to over 100 and no fragrance to strongly so. Some are short, some very tall, some repeat flowering, some once only. In general, though, one could say that they are tough roses that will grow well in less than ideal conditions, although beauty and fragrance are not strong characters in most.
Good examples of earlier varieties are the Fruhlings series, including Fruhlingsgold as well as Nevada and Scarlet Fire. The German rose breeders Kordes have been responsible for many excellent varieties. More recent varieties are excellent garden plants.