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I read that wallet/account is not the same as bitcoin address. The address can be used only for single transaction. How do you receive donations on your address? Are wallet addresses unique for the network so that I can generate my wallet in one service, eg. blockchain.info, and used it in different service? How do you call the address of the wallet if not address?
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When simplified a lot, Bitcoin address can be seen as a public / private key pair.
Wallet can contain several addresses. There is no relationship back from the address to an wallet - basically wallet is just a list of addresses known to you. Wallet also knows the private key to these addresses which it is using when you send Bitcoins out from a wallet.
Usually you publish your public part of Bitcoin address on a web page, so others can send Bitcoins to this address.
You can use the same address to receive multiple transactions. This is how people usually receive donations. They just publish their Bitcoin address on a web page. With some programming, you can generate a new address for every donation if you want to maintain more privacy.
Some Bitcoin wallets allow you to import/export addresses and their keys, so in theory you can move addresses to different wallet: https://en.bitcoin.it/wiki/How_to_import_private_keys - however not all wallets support this, this is advanced operation and normally not recommended for a normal user. So if you want to migrate to a different wallet (service/software) I suggest you just start using it, send bitcoins from old wallet to new one, generate new receiving Bitcoin address in the new wallet and publish this address.
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I read that wallet/account is not the same as bitcoin address.
A bitcoin-wallet is only a way to store multiple private keys (corresponding to multiple bitcoin-address).
The address can be used only for single transaction. How do you receive donations on your address?
Addresses can be used for an infinite number of transactions. If you put a donation button somewhere referring to an bitcoin-address of yours, all those donations will get paid to this one.
Are wallet addresses unique for the network so that I can generate my wallet in one service, eg. blockchain.info, and used it in different service?
With a wallet (like the bitcoin-core client) you can generate new bitcoin-addresses as well as using other services like blockchain.info. Those addresses are unique and by exporting/importing the private key you can use those addresses in other services as well.
