On 1st of august we'll probably witness of Bitcoin chain split into two chains: BTC and BCC. It means, that chain which we actually have will be copied into another - identical chain. However, since the split - both chains will start to differ from each other. For example, if we have 1 BTC at 'aaaa' address, we will have 1 BTC at 'aaaa' address and 1 BCC at the same address. From this moment we can send both BCC and BTC to any address from 'aaaa' address. For example, we can send 1 BTC to 'bbbb' and 1 BCC to 'cccc'. Since then, the two chains are already different because we have made two different transactions.

Exchanges

Depending on the policy of a particular exchange - these take different positions. One declare support for both chains, but most will probably support only one chain (BTC Chain). Most exchanges declare that before August 1 your funds should be taken out of the exchange (if one wishes to be the holder of funds on both chains), otherwise it will only be possible to operate the funds only within one chain. I personally do not advise holding BTC on any exchange before 1 August.
Why should we care to be the owners of funds on both chains? Because both BTC and BCC will have some value. Despite the opinion that one of the chains will fall, or it will be worthless - the truth is that the person holding 1 BTC and 1 BCC will be richer than a person with only 1 BTC.

Wallets

Unfortunately, wallet developers also declare support for a single chain. Electrum creators for example have issued a statement that the transactions that will be made with their wallet on the BCC chain will be treated as invalid.

An official list of wallets and exchanges supporting the BCC chain is available at: https://www.bitcoincash.org/.

And so we see that at 26.07 we have 3 wallets, which signal BCC support: BitcoinABC, ElectrumCash and Ledger.

What does it mean?

This means that in order to send your coin to another address (such as the viaBTC, which supports this chain), we need one of the above wallets. The ElectrumCash wallet is probably a copy of the original Electrum wallet just adapted to the BCC chain.

What should we do if we currently have Bitcoins on a wallet that does not support BCC?

We are still safe. As long as we use a wallet that gives direct or indirect(seed) access to private keys, we still have control over the coins within both chains. In this case, to send BCC coins (for example, to the exchange) you must have a wallet on which you hold the coins to export the private key and then import it into the BCC support wallet (BitcoinABC, ElectrumCash, Ledger). Then, from this wallet, you have to send coins, for example, to a exchange. I do not recommend keeping funds on August 1 at: exchanges and online wallets ( eg blockchain.info).

Important !!!

If you send funds within one chain, make sure that both the wallet you use and the exchange support the chain. This is particularly important with the BCC chain. On the exchange you need to make sure that the address to which you send bitcoins is the BCC address.

Reply attack

Reply attack is an attack that has taken place in the past on the ETH chain, at times when it was split (split into ETH and ETC). The point is that after splitting, there is a likelihood that a valid transaction made within a single chain (eg BTC) can be copied and announced within the second chain (eg BCC). Finally, the same transaction will be executed on both chains. Example: We are sending 1 BCC from 'aaaa' address to 'cccc'. We check the status of the 'cccc' address on the BCC chain and have 1 BCC. We check the status of the 'cccc' address on the BTC chain and we also have 1 BTC! In short - the point is that the same transaction will be repeated on the other chain. For a transaction to be repeated, the history of the address must match both chains. So to defend against an attack, you need to change the history of the address on the chain.

BCC developers have announced that they have implemented a solution that will not allow attack. However, to be 100% sure I prepared a mini-instruction on how to defend yourself:

1. If you have a BTC on a paper wallet or another wallet, you need to import a private key into a wallet with BCC support (eg ElectrumCash).

2. Generate a unique address (for example, at bitaddress.org), and send the coin within the BCC chain using this wallet eg ElectrumCash. Let's call this new address 'dddd'.

3. At this point it is necessary to check whether someone (or some bot) copied the transaction and used a reply attack (enter 'dddd' address at blockchain.info and check if attack succeeded).

4. If the attack succeeded, it means that all BCC safeguards and promises did not work out and panic would explode :D Fortunately, first transaction we made, we made to another address to which we have a key ('dddd' address from point 2), and We still have control over the coins within both chains. In that case, wait patiently until the situation is clear. At this time, a lot of people will lose their coins from the second chain. We can repeat points 1-4 from time to time (remember that every attempt involves a transaction fee on the network) until no one copies our transaction, then go to step 5.

5. Nobody has copied our transaction, so to definitely separate the two chains we generate another address (eg bitaddress.org). Let's call it 'eeee'. In our favorite wallet supporting the BTC chain (eg regular Electrum) we make a transaction on the newly created 'eeee' address. At this point we have BTC coins at the 'eeee' address and the BCC at the 'dddd' address. No one is able to repeat any of our future transactions, because the history of both addresses within the two chains is already different.

From now on we can safely treat both chains as separate cryptocurrencies.

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UPDATE !!!

I was informed that more or less at the time of writing the post Electrum officially responded to the new creation ElectrumCash:

https://electrum.org/bcc.txt

It follows that the makers of ElectrumCash without the knowledge of Electrum have appropriated the logo and name of the product, and that the official creators of Electrum is cut off from ElectrumCash. Because of this information, we should take additional caution regarding BCC wallets. I suggest starting to test a solution for small amounts, or wait a few days with any transactions. So what do to to keep uor BTC safe on Electrum? Electrum developers says:

"Therefore, after the BCC fork, but before you enter a seed or
private key in a BCC wallet, you should move all your funds to a new
Electrum wallet, with a new seed. You will still be able to use the
old seed or private key with BCC, because BCC has replay
protection. Wait until your funds are confirmed in your new Bitcoin
wallet, before you enter the old private key in a BCC wallet. This
will protect your BTC funds from rogue/untrusted software."