Often they are a set of numbers representing the same kind of thing, like body weight. If the numbers are the same, the thing is called a constant . I said usually a set of numbers, because some data are a set of labels, names, or levels. Again, when these labels represent the same kind of thing, that thing is a variable. For example, the labels male and female are values for the variable sex.
Numeric variables come in several varieties.
Things like height and weight are the usual kind. These can have just about any value to as many decimal places as we like, so we call them continuous . An example of a variable that is not continuous is a count , such as the number of injuries a person has experienced. One other kind of variable can't decide whether it's numeric or nominal. A good example is competitive level, with values of novice, club, national, international.