Travel options

I'm studying the public transportation options around London. There's a tube, which I think is mostly underground. Then there's a national rail, and a bus system. It looks like you can pay per ride, which seems to be the most expensive way. Then there's an "Oyster" card, which seems to be like our MetroCard in DC. The Oyster card gives you a small discount and you fill it up with more money as necessary. Then, it gets tricky, there's a "travel card" that allows for unlimited rides in a certain time period. Apparently it goes on the Oyster card. I think I'll try a week or a month travel card and make sure I make enough trips to make it worth it, I think the cost in on the range of $300-400 per month.