Ripple itself is great, a side feature of it is the currency XRP, and the ability to trade within the client - this text focusses purely on a quick intro to trading within the client.
Basic:Currency like USD and BTC can be traded within the Ripple client by using Gateways - such as Bitstamp and Weex. In short you add currency to your gateway account, then withdraw it to Ripple, trade within Ripple, then deposit from ripple back to the gateway (and elsewhere) as you see fit.
WhyLike any other digital currency, it's value can fluctuate up and down, on a per currency basis - it's roughly tied to the $USD and this provides two important properties to the trader (a) arbitrage ability (b) a comparatively stable home for digital currency whilst BTC / ***coin markets fluctuate.
A couple of weeks ago one XRP was worth $0.0009 and now it's worth roughly $0.00138 - a rise of 43.6%. If it goes parity with the cent that's a 935% rise in value. Even a conservative estimate of $0.027 USD per XRP gives a 2695% rise.
Seeing the markets:The client:
http://ripple.com/client1) Sign up
2) Contacts -> add contact (the +) Name: Bitstamp ... Address: rvYAfWj5gh67oV6fW32ZzP3Aw4Eubs59B
3) Advanced -> Trade ... click "change issuer" and start typing bitstamp or rv.. hit "confirm" .. hit "order book"
4) Chance currency pairs and repeat the (3) for each
Using the markets:You'll need to get some USD/BTC/Whatever in to Ripple, to do this you need to trust the gateway (say bitstamp) and then withdraw from the gateway to your ripple account.
1) Advanced -> Trust (add +) .. enter user Bitstamp, then the amount and currency - typically set this high, certainly higher than the amount you want to put in to ripple. You must repeat this for each currency you want to use.
2) In Bitstamp (or whatever gateway) hit withdraw -> ripple, then withdraw to your ripples address.. within seconds it'll be in your ripple account.
3) make offers to buy and sell currency.
Caveats:You need a base level of XRP to add trust, and to make offers, it's good to keep min
500-1000 200 XRP in your account at all times - if you don't have any and want to get started, ask in #ripple on freenode and I'm sure somebody will lend you some to get started, or sell you some for defacto market rate.
Disclaimers:I don't work for Ripple, Opencoin, or anybody, I'm unaffiliated and have no business interests other than a spot of typical side trading in my spare time.