In Java servlets, a session is a way to maintain stateful information about a user across multiple requests. When a user visits a website, the server creates a new session object to keep track of that user's interactions with the website. The session object is associated with a unique session ID, which is typically stored as a cookie in the user's web browser.

To work with sessions in a servlet, you can use the HttpSession interface, which provides methods for creating, retrieving, and manipulating session objects. Here are some common tasks that you can perform with sessions in a servlet:

1. Creating a session - To create a new session, you can call the HttpServletRequest object's getSession() method. If a session object already exists for the user, this method will return the existing object; otherwise, it will create a new session object and return it.

2. Setting and getting session attributes - Once you have a session object, you can set and get attributes on it using the setAttribute() and getAttribute() methods, respectively. Session attributes are stored as key-value pairs and can be any type of object.

3. Invalidating a session - To end a user's session, you can call the invalidate() method on the HttpSession object. This will destroy the session object and remove the session ID cookie from the user's browser.

4. Session timeouts - Sessions have a timeout value, which is the amount of time that the session will remain active without any activity from the user. The default timeout value is usually 30 minutes, but this can be configured in the web application deployment descriptor (web.xml) or programmatically using the setMaxInactiveInterval() method.

Using sessions in a servlet can be a powerful way to maintain stateful information about a user, such as user preferences, shopping cart contents, or login credentials. However, it's important to use sessions judiciously and be mindful of the security and performance implications of storing user data on the server.