The only requirements of nominal variables is that they are exhaustive and exclusive. Exclusive means that the categories don t overlap. If you were asking someone's religion, it would be inappropriate to include Christian and Catholic as categories because they overlap. Exhaustive means that every possible answer is included. The second level of measurement is ordinal.
Ordinal variables are exhaustive and exclusive plus they have an inherent order.
If some one gave you a list of the possible answers, you would know what order they go in and one end of the order would be clearly higher than the other. Low, Medium and High is an example. Zodiacal sign is not because no sign is inherently higher. The third level of measurement is interval. Like the previous levels, it is exhaustive, exclusive and has an inherent order.