The Basilica dei San Marco, or St. Mark's Basilica, is probably the first stop in Venice for most visitors. Lines to get in are incredibly long, but if you go later in the day you're less likely to enjoy the international queue. This picture was taken from the clock tower. Notice the small platforms on the ground in the lower right- they are for visitors to stand on during flooding.
The first St. Mark's was built in 828 to house the recently stolen relics of Mark the Evangelist. The current Basilica was constructed between the 11th and 16th centuries.
The Triumplan Quadriga, or Horses of St. Mark's, were stolen from the Hippodrome in Constantinople in 1204 and were installed on the Basilica in 1254. These are actually copies outside today, the originals are inside for viewing.
There is (of course) no photography allowed inside the Basilica, so you only get to enjoy the exterior. The interior is a vast, elaborate display of frescoes and gold leaf. You're slightly rushed through in a specific route, and to be honest we found it the least enjoyable of all the churches.