To cancel an open order, first make sure you're viewing the market on which your order was placed (e.g. BTC-USD, LTC-BTC, etc). Your open orders will be listed in the Open Orders panel in the lower middle of the trading dashboard. Hover the order that you desire to cancel and click the "x" to the right hand side of the order.
Yes, however for best performance of the GDAX web interface and API we recommend using a reliable and fast connection.
We do offer margin trading! For more details, please see the Intro to Margin article.
Placing a limit order lets you decide what price you wish to buy or sell at. Your order will be added to the order book and will execute when someone submits a matching order.
See also: Entering Market, Limit and Stop orders.
The order book is a list of all open (unfilled) buy and sell orders. You can buy or sell digital currency at any price listed.
Newly placed orders first enter the order book as an "open order". When some of your "open order" matches with another order, it becomes a "partially matched order". When all of your order matches with another user's order, it becomes a "fully matched order" or "completed order". Completed orders are removed from the order book. Open orders can also be cancelled.
GDAX does not charge any transfer fees for moving funds between your Coinbase account and GDAX account. See also: How to transfer funds.
You have open orders. The funds used to place your orders are considered to be on hold and not part of your available balance. See also: How can I view current holds for open orders on GDAX?