To answer your question in a personal non-googly kind of way:
It's a celebration that happens the day before Ash Wednesday all across the Gulf Coast, from Pensacola, FL to Galveston, TX. New Orleans is obviously the most well known.
As far as the cake goes, the cakes are very sugary and there is a tiny plastic baby inside. Everyone cuts a slice and if you get the baby in your slice, you have to buy the next cake. This is usually a Friday thing at my office.
The baby is there in the middle:
It's basicaly different parades that move through the town. People dress up on the floats, play awesome music, throw beads and other goodies to everyone on the route. Yes, there is a lot of drinking. It's just a big huge party. Parades usually happen every weekend for weeks before Mardi Gras.
On Mardi Gras day, we have about 5 parades here on the MS coast. So, it's an all day affair. People shut down businesses (except mine) and party. Depending on where you stand or set up, it can be very family friendly, or very NOT family friendly.
Some people get to the parade routes early in the morning and set up, similar to a tailgate party. It's fun to walk the parade route before the parades, stopping along the way, talking to people, and eating their food.
Oh and Krewes, every parade is run by a Krewe. Most Krewe's have a ball sometime after the start of the year. You dress in ball attire and party all night. Each Krewe has a king and queen and court every year.
That's about it for basic Mardi Gras info.
Laissez les bons temps rouler!