As the bible says just after that, the seven heads are seven kings (or kingdoms).

I believe these seven heads represent the kingdoms that had Israel under bondage and tried to wipe them out througout its history.

Egypt
Assyria
Babylon
Persia
Greece

These are the five that were fallen at the time of the writing of the Book of Revelation

The one that is was the Roman Empire that was in existence when John was around.

The Seventh Head is the coming world leader/government.

The body which is also the 8th king is actually the Spirit of AntiChrist that was the motivating force behind all of these Kings to try and destroy the children of Israel.
Another interesting fact is that we know there have been several attempts to do that in history such as the Spanish Inquisition and the Holocaust.  The difference is that Israel wasn't in the promised land like they were with all but Egypt (but they still were a corporate body in Egypt).

We know that Egypt enslaved the children of Israel and that Pharoah had a decree to kill all the male children that were born(Exodus).
We know that Assyria removed the northern kingdom and transplanted them somewhere else and attempted to remove their Identity as the Children of Israel (hence the Ten Lost Tribes)
We know that Babylon enslaved what remains of Israel (the southern kingdom) and forced everyone to bow to the Golden Image (except Shadrach, Meshach, and Abed-Nego...Where was Daniel???)
We know that the Persians were going to Kill all the Jews if it were not for Esther.  Also Cyrus made a decree to re-establish Judah (under his control).
Also the Greeks controlled the jews and nearly destroyed them again with Antiochus Epiphanes who desecrated the temple and slaughtered a pig on the alter.
Finally we all know too well that the Romans controlled Judea and in 70 AD destroyed them and scattered the survivors.
The final King/Kingdom will likewise try to destroy the Jews and remove them from their promise land.

I hope that helps explain things.