Java provides a number of simple data types like int, float and String. However very often the data you want to work with is not an int, a float or a String. Classes let programmers define their own more complicated data types.
For instance suppose your program needs to keep a database of web sites. Each site has a name, a URL, and a description. In traditional programming languages you'd have three different String variables for each web site. With a class you combine these into one thing like this.
public class website {
public String name;
public String url;
public String description;
}