What are the main types of tags in JSP?

JSP uses three main types of tags:

Scriptlets: Allow you to insert Java code directly into an HTML page. Used for business logic execution or data access.
```
// Java code insertion
<%
    String user = "Guest";
    System.out.println("Hello, " + user);
%>
```

Declarations: Used to declare variables and methods at the JSP level. These declarations are accessible from any scriptlet on the page.

// Declaration of a variable or method
<%!
    private int counter = 0;
    public String getMessage() { return "This is a declaration."; }
%>

Expressions: Used to output the value of variables or expression results to the page. The result is converted to a string and inserted into HTML.
```
// Output the value of a variable or expression
<%=
    new java.util.Date() // Output current date
%>
```

In addition to the main types, there are also:

Directives: Control the behavior of the JSP container during the translation of the JSP page into a servlet. For example, page, include, taglib.
```
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
```
Action Tags: Predefined XML elements that perform standard tasks such as including other resources or creating JavaBean objects. Examples include jsp:include, jsp:forward, jsp:useBean.
```
<jsp:include page="header.jsp" />
```
Custom Tags: Allow developers to define their own tags for reusing functionality and improving JSP readability. Used with the taglib directive.
```
<%-- Example of using a custom tag --%>
<%@ taglib prefix="my" uri="/WEB-INF/mytld" %>
<my:hello name="World"/>
```