VaR (Value at Risk) is a core concept in risk management. Describe the primary methods for calculating VaR, outlining their characteristics and typical use cases.

你好，在风险管理中，VaR（Value at Risk）是一个核心概念。请你简要介绍一下计算VaR的几种主要方法，并说明它们各自的特点或适用场景。