Long Position and Short Position
A long position is opened by buying a security, under the assumption that the security will increase in value. It is closed by selling the security.
A short position is opened by borrowing and then selling a security, under the assumption that the security will decrease in value. It is closed by buying the security and returning it to the lender.
To fill a buy or sell order is to execute the order by matching it with one or more orders of the opposing type. Buy orders are filled by matching with sell orders (and vice versa).
The order book is a list of current buy and sell orders, which is used by an exchange to fill orders.
The order book consists of both orders to buy or sell at a fixed price ("limit" orders) and orders to buy or sell at the best available price ("market" orders). But since market orders only appear in the order book momentarily, they aren't shown in the publicly viewable order book (i.e. the order book shown under "Market Data" on Kraken).
The order book is visible at https://www.kraken.com/charts
Here's an example:
Bid and Ask
A bid is an order to buy in the order book.
An ask is an order to sell in the order book.
The bid depth at a given price is the cumulative volume of current buy orders on the book at that price or higher. The ask depth at a given price is the cumulative volume of current sell orders on the book at that price or lower.
The bid depth at a given price indicates the volume that can be sold before the value of the security goes below the given price. The ask depth at a given price indicates the volume that can be bought before the value of the security goes above the given price.
The bid/ask depth chart under "Market Data" only shows the bid and ask depth of limit orders on the book.
The bid/ask spread is the difference in price between the highest bid and the lowest ask on the order book.
The bid/ask spread chart under "Market Data" only shows the spread between the highest limit buy order and the lowest limit sell order (plotted over time). A market order will fill in this gap if there are matching market orders of the opposing type sufficient to fill it. Otherwise it will fill, at least in part, using limit orders of the opposing type.
Spread data can be seen in near-real time on Kraken here: https://www.kraken.com/charts
The above example shows the top bid (Buying) 4.615 bitcoin for 660.95 euros. The top ask (Selling) in the above example is for 6 bitcoins at 660.999 euro. The spread is the difference between the top bid and top ask. In this case, it's 4.9 cents.
In a currency pair, the value of the base currency is displayed relative to the quote currency. The currency pair is priced in terms of the quote currency and the price indicates how much of the quote currency is required to buy one unit of the base currency. For example, a price of $110 for XBT/USD means that $110 USD is required to purchase 1 XBT. In currency pair trading on Kraken, a pair can effectively be a single trading instrument that is opened as a position by buying (going long) or selling (going short) the pair.
Base Currency and Quote Currency
Base currency is the first of a currency pair. For example, in the currency pair XBT/USD, XBT is the base currency.
Quote currency is the second of a currency pair. For example, in the currency pair XBT/USD, USD is the quote currency.
Untouched and Touched Orders
An "untouched" open order is an unfilled order. A "touched" open order is an order that is partially, but not completely, filled. A completely filled order will be listed as closed.
Under the "Trade" > "Overview" tab on the account screen you will find your account ledger. The ledger is a history showing how your account balances have changed as a result of deposits, withdrawals and trades. The "Fee" column shows fees that have been deducted from your currency balances. The "Amount" column shows other deductions from and credits to your currency balances. The "Type" column indicates what type of action changed the balances (Deposit, Withdrawal, Trade or Margin Trade).
For example, suppose you create a limit buy order for 10 XBT at $100. When this order fills, ledger entries will appear showing a deduction of $1000 from your USD balance, the fees deducted for the trade (in the "Fees" column), and a credit of 10 XBT to your XBT balance.
If an order opens a currency pair position (a margin trade), only a trade fee will be deducted until the position is closed. When the position is closed, the profit or loss will be credited to or deducted from the balances. For example, if you open a $1000 long position in XBT/USD at $100, only the trade fee will appear in the ledger. If you later close this position for $1200, the $200 profit will be reflected as a $200 credit to your USD balance (but of course the trade fee will also be deducted).
Please note that if several matching orders in the order book are required to fill an order, this will be reflected in the ledger by several entries. Clicking on the ledger ID number of a ledger entry will show you which trade and order IDs are associated with the ledger entry.
Reserve vs non-Reserve Currency
A reserve currency is one that can be withdrawn from or deposited to your account. A non-reserve currency can only be margin traded and cannot be deposited or withdrawn.