The short describtion:
DRBD is a Distributed Replicated filesystem, RAID 1 over TCP/IP.
You have one part on machine one and the other at machine two.
You can only mount at one machine and it has to be the primary.

Heartbeat (HA) is a tool to check if a machine there is running a bunch of services/deamons goes offline, then another node in the HA starts the deamons/services up again at the same IP.

Some usefull links for deeper documentation :

DRBD : Distributed Replicated Block Device for Linux.
DRBD FAQ : Read it before you start !
Heartbeat (HA) : The High-Availability Linux Project.