For free, you will typically get access to programs from channels that are part of your subscription, along with various local and some free movies. There are also many PPV movies. If you have kids, you definitely want to set up a Purchase PIN. Even without kids, I still have one (yes, it's just a simple 1234), to make it very obvious something will cost $$.
Since you haven't used Ondemand before. Here is some information about Ondemand. A rule of thumb with Ondemand. If you subscribe to it you will have access to watch it Ondemand.