Anyone who has seen the movie Gladiator will know that
the Colosseum in Rome was used for gladiator contests. This amphitheatre was also used for public speaking, animal hunting, religious ceremonies and drama performances. The construction was completed in 80AD under the reign of Emperor Titus.

The Colosseum in Rome from the outside
Nowadays it is one of Europe's biggest tourist attractions. From the photos you'll be able to see that a lot of the building has fallen, this was caused by earthquakes.
The official
website for tourism in Rome has a lot of useful information. It is best to visit the website for updated opening times and tickets prices. My ticket last summer was €12 (adult ticket). If you are planning to visit the Colosseum I would definitely recommend purchasing your ticket beforehand, either online through this
website link, or take a trip to the venue the day before to buy tickets
Tickets can be purchased on the day but the queue for the tickets is very long and if you go in summer this can be an unfortunate combination. The other option is to purchase a guided tour ticket from the guides who are dotted outside the venue (don't worry they will find you). They are able to cut the ticket queue so you'll only have to queue to get in to the actual venue. The disadvantage of taking a guided tour is that you may feel that your opportunity to explore and take photos is restricted. Also sometimes it can be hard to hear the guides amongst the tourists.
Part of the Hypogeum in the Colosseum
Inside the Colosseum there are many great sights to see. One of them is the Hypogeum, this is the rooms, tunnels and passages that can be seen underneath the Colosseum. This area had many uses for example for the gladiators and as animal pens.

The Hypogeum can be seen underneath the Colosseum
The Arch of Constantine can be seen as you look out from the Colosseum.

The Arch of Constantine
If you do decide to go in summer take plenty of water, without a 'roof' type structure the Colosseum gets very hot with the direct sunlight exposure.