In CSS, a class is a style definition that is a subset of another style definition. For example, you could define a class of the paragraph tag for contact information, to make contact information display centered in large, red, bold text.
In object-oriented programming, a class is a user-defined type (as opposed to int, which is a built-in type). Classes have defined characteristics and behaviors, known as attributes and methods. A class serves as a blueprint for an object.
A POSIX character class is a code that represents a set of characters; for example alpha represents all alphabetic characters ([A-Za-z]).