This church is old, and like many old churches, it has a baptistry across from it. (The baptistry is the round thing above.) Way back when, you weren't allowed to enter the church unless you were baptised, which is why the baptistry is a separate building. On the front of this building used to be the gates of paradise, a historical piece of artwork. It has since been replaced with a replica (pictured) and the real thing has been moved to a place where it can't be affected by weather or pigeons.
This sign shows that the church was commissioned by a cardinal (hence the green cap). If it has been commissioned by the pope, it would have the pope's hat and the shield would have keys on either side.
The dome itself was build to compete with another city- Florence wanted a big and better church. The church itself is the third largest in the world, and I believe the dome is the largest dome in the world. Even more impressive, it was built from the bottom up, since they didn't have fancy cranes and such when it was built.
The church itself is pretty, but my companions and I didn't like it very much. It was obviously just built for show and not to be a real church. Most churches that allowed tourists in at least had an area where you could sit down and pray. This didn't exist at all in this church. There was no where where we could stop and pray, which led us to leave and seek out a smaller church a bit away.