The market is exactly that, a huge always growing marketplace full of applications for your phone. Some are paid (yest cheap costs) and most are free. Since everyone is different we all have different applications that work well for us personally. There are some amazing apps on the market, and there are also a lot of crappy ones, but thats what you get with an open platform.
Most importantly, you get 24 hours to 'try out' a paid application and if you don't find it useful you can uninstall and refund within the market. All costs are connected to your Google account. The market rocks.