http:// stand for hyper text transfer protocal. It basically defines the protocol you want to use to transfer data. Http: is generally associated with TCP port 80.
Other protocls include https which is http secure
ftp or file transfer protocol and i believe one that is basically never used called gopher.
WWW on the other hand has nothing to do with protocols. It stands for world wide web and is part of the address.
however not all addresses have www.
so to recap.
HTTP:// defines a protocal while www is part of the adress.
Think of it as this. When you send a letter the WWW is part of the mailing address and the HTTP:// could be representative for airmail. FTP could be express and HTTPS could be considered registered mail.
EDIT just to be complete.
FTP is generally associated with TCP port 21 and HTTPS is generally associated with TCP port 443
