Sorry for the noob question (but I guess thats why this forum is here...). When I see $100 MtGox USD quoted is that a USD100 worth of Bitcoin (ie around 7.35 BTC). ?
$100 MTGUSD refers to $100 worth of USD funds held in an account with Mt. Gox.
At one point in time, Mt. Gox had a Send Money method that would let one trader transfer any amount of USD funds from the trader's own account to any another account on Mt. Gox. That feature was discontinued just about a month ago when Mt. Gox redeemable codes / vouchers were introduced.
The redeemable codes provide nearly the same function. The codes are like a stored value card. Think like a Green Dot MoneyPak.
Once you create a voucher for whatever amount you specify your balance is reduced by that amount and you are given a code.
You can then provide that code to anyone with a Mt. Gox account and when the recipient redeems (claims) your voucher code, the recipient's account gets the funds from the voucher.
When you "withdraw" funds to a voucher, you specify whether the voucher should be for bitcoin (BTC) or USD funds.
These voucher codes are useful for transferring USD without having to withdraw funds to Dwolla or to a bank first. Because the funds are available instantly, you will see MTGox USD vouchers used in over the counter trading. For example, here someone is selling MTGUSD in exchange for PayPal
Additionally, the BTC vouchers might be useful for sending bitcoins to someone who does not yet have a bitcoin address.
