The wedding cake is the first thing your guests notice when they walk into the Reception hall. The cake sets the tone of the wedding as formal or informal. In ancient times weddings took place after the harvest and cakes were considered a symbol of good luck. Wedding cakes gave good luck to the newlyweds, and passing out pieces of cake was a way of sharing that luck with friends and neighbors.
The traditional wedding cake of three or more round stacked layers, is what comes to mind when most people think "wedding cake" However, your wedding cake can be any shape or style you wish. I have seen all kinds of cakes used for wedding cakes, from multi-tiered heart shaped cakes with several smaller hearts on pedestals attached to the main tower by cascading ribbons, and plastic bridges. It was really beautiful.
I also know a couple who met at a hockey game. Their wedding cake was a sheet cake decorated to look like a hockey arena. Obviously it was a very informal wedding, and a second marriage for both. A sheet cake can be decorated with elegance, for an informal wedding. It is your wedding, you can have any kind of cake you want.
Most brides show up for the first meeting with a cake designer/decorator, with an image in her head of the perfect wedding cake, that she may have dreamed about since childhood. The designer will do everything in her power to bring that image into reality. Go to the meeting the approximate number of guests, your color scheme, a sketch however primitive of what you want your cake to look like and possibly a picture from a magazine that is close to what you want the cake to look like. The designer will also want to know what flavor you want the cake and icing to be. They may even have samples for you to taste.
If you want a formal wedding but still want to add a touch of whimsy have fun with the grooms cake. The wedding cake can be as elegant as you want, but have the groom's cake reflect something about the groom, his hobbies, job etc. Does he like Nascar have a race car, the options are endless. If another special family occasion is the same day as your wedding, try to recognize it some way.
A friend of mine got married on her father-in-law's birthday, so she had the top layer of her wedding cake decorated as a birthday cake for him. He was so delighted. All of these kinds of things need to be discussed with the cake designer, well in advance of the big day. You can get wedding cakes at most bakeries, but I'd suggest looking for one whose specialty is wedding cakes. You don't have to limit yourself to the traditional cake. I knew a bride whose wedding cake consisted of hundreds of filled cupcakes, artfully arranged to resemble the traditional wedding cake, each of the five layers had a different type of filling. It was amazing!