You can reply to a message directly from the mailing list web interface. To do this, click on the 'Reply' button on the top right of the message.
Be careful: after clicking on the 'Reply' button, you will not be able to change the recipient anymore; thus be careful when choosing between the message's sender and the list (i.e. all subscribers)!
This method is not very flexible: from the mailing list web interface, you can not add or change some recipients, add an attachment or format the messages you send. Thus you might prefer to use an email client or a webmail.