The `java.util.Calendar` class in Java is an abstract class that provides methods for converting between a specific instant in time and a set of calendar fields, such as year, month, day, hour, and minute. It allows developers to perform date and time calculations, such as adding or subtracting days, months, or years, and can be used to manipulate dates in various formats. `Calendar` is part of the Java Collections Framework and is often used in conjunction with other date and time classes, such as `java.util.Date` and `java.time.LocalDateTime`. It provides a way to handle time zones and locale-specific date formats, making it a versatile tool for managing dates in Java applications.