Webmail is a system for the management of emails. Emails can be written, sent, saved, forwared, etc. - in fact everything an email system provides. Webmail contains mailboxes, folders, address book, filters, signature, GNU privacy guard, search mode, etc.
The advantage of Webmail: it is web based. That means that no special client is necessary. Emails can be downloaded from any computer which is connected to the Internet, using a browser and a login.