I see that there is the "receive coins" tab, and this is the number/account (what do you call this?
) I share if I want to GET coins.
Most people call it a "bitcoin address", or if it's obvious from context that you are talking about bitcoin, simply an address.
Mine is : 1QCvEe6xU6Dsh83bxEaEsh8ftGnoqUvgoH
You can have more than one address in a wallet. You can have as many as you like. You might want to create a different address to give to each person/company/entity that might send you bitcoins. That way when you receive the bitcoins you'll be able to tell where they came from since you'll be able to see what address they were sent to. For that matter, it is generally a good practice to use a new bitcoin address for every transaction when possible. This helps to increase your privacy and anonymity. To get an additional address in your wallet, you can click the "New Address" button in the "Receive Coins" tab.
However, I get confused as soon as I start to think about my "account"
Do I have a account number, . or just send and receive address?
Just send and receive addresses. There is no account number.
There is a back up option, it makes a "DAT" file, is that the file I need to keep track of.
Yes.
If I hve a backed up version of the dat file, are there risks I should know about.. is it one time back up , .. or after each trasaction?
The backup stores all your current bitcoin addresses (and their respective hidden "private keys") as well as the next 100 addresses that you will use. Every time you click the "New Address" button, it draws from this pool of 100 addresses. Also, every time you create a transaction to send bitcoins somewhere, it draws from this pool of 100 addresses and moves some of your bitcoins to the new "hidden" address that it doesn't bother telling you about, but keeps track of for you in the ".dat" file. You don't see the new address in the "Receive Coins" tab since you didn't request it, but since the wallet knows about it in the ".dat" file, it includes the coins that are associated with the address when it displays your balance.
As such, it is a good idea to get in the habit of backing up on a regular basis. How often really depends on how often you create addresses in the "Receive Coins" tab and how often you create transactions to send bitcoins. Sum up the approximate number of addresses you've "created" using the "New Address" button plus the number of transactions you've sent all since your last backup, and if that sum is larger than 25 consider creating a new backup. Then keep the 4 most recent backups somewhere safe. This way you've got at least three copies of backups that should contain access to all your bitcoins. If one backup becomes damaged or otherwise unrecoverable, you'll have a slightly older backup you can revert to.
Note, if you add a password to your wallet (or change your password on your wallet), you should IMMEDIATELY backup. The process of adding or modifying a password on the wallet discards the entire pre-generated pool of 100 address and generates 100 new addresses using the new password. This means if you don't immediately backup, then you won't have any of the new addresses in any of your current backups.
... how do I "USE" the dat file, I don't see a option to "import dat file" or "restore from back up" in bitcoin -qt
You shut down Bitcoin-Qt. Then you find the directory on your hard drive where the wallet stores its wallet.dat file (it is different in each operating system). You place a copy of your backup file in that directory, and then start Bitcoin-Qt up again.
Oh, and 1 more question, . how do I find out the address that they "came" from .. or is that even possible ... seems to me that bit coin is "1 sided" in so far as you can see receiving addresses, be not "sending" address is this right?
In general bitcoins don't really have a "came from" address. With technical knowledge it is possible to parse the blockchain and find the transaction output that was spent. By looking at the details of that transaction, you can see what address (or addresses) the bitcoin value was previously associated with. Others with soe technical knowledge have already done the parsing for you and created websites to allow you to look up data from the blockchain (blockexplorer.com and blockchain.info). If you search for the transactionID at blockchain.info, you can see the previous addresses.
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