what is the different between an octopus card and a tourist day pass ?

The former card can be constantly used as long as there is enough deposit money in the card. But the latter one is valid within a month and can be only uesd in one day for the unlimited travel on most of the MTRs.
Additionally, Octopus Card can be used on MTR, bus and Airport Express, but the tourist day pass can only be used on MTR.