The word “russet” refers to the rough and dull skin found on some apple varieties, older varieties in particular. This rarely found on modern apples because it has been bred out over many decades.
In the majority of cases a russet apple will not have the crunchy flesh of other apples, it will be be more dense and moist. In some cases, Egremont Russet is a good example, the flavour is far more developed and pronounced.
There are several different varieties, and whilst they are not necessarily related to each other, they tend to share some common characteristics – notably the attractive russet colouring, a sweeter flavour, and flesh which is firm rather than crunchy.