Now that we've sent the bitcoins from the faucet, let's check whether Bitcoin Core can see the transaction. To do that, we'll need to list the available UTXOs in both clients, using the listunspent RPC command.

With Bitcoin Core running, run the following command in your Terminal window:

bitcoin-cli listunspent

This will return the following result:

[{ }]

Initially, listunpsnet returns an empty result, because Bitcoin Core hasn't yet finished syncing the blockchain, which takes time (a few hours). For this reason, we will use Electrum instead of Bitcoin Core for the remainder of this guide, as it avoids us waiting for hours to see the received bitcoins.

However, we will go back using Bitcoin Core from time to time, as it has a powerful command line to deal with raw transactions.

Now run the same command for Electrum, as follows:

electrum --testnet listunspent

We will get a list of available entries, such as the following:

The previous command's output shows that we have a single available transaction received from the faucet, uniquely identified by its hash (prevout_hash field), with 1.1 Bitcoins.

More precisely, we have an available unspent transaction output from a previous transaction, which can be used as an input for the transaction we are willing to build, as follows:

In Bitcoin, transactions spend outputs from prior transactions, and generate new outputs that can be spent by transactions in the future. In fact, users move funds solely by spending UTXOs.

The previous diagram shows that the transaction (Transaction C) we received from the faucet consumes as inputs an existing output(output 1) created earlier by an old transaction. The same transaction creates two outputs: one for us (output 1), and the other returns back the change (output 0). The reason for this is that transaction outputs must be fully spent. 

Unlike what you might have expected, in bitcoin, transactions don't update a global user balance (the account/balance model). Instead, they move bitcoins between one or more inputs and outputs (the UTXO model). The total balance is calculated by the bitcoin client as the sum of the values transferred by the received unspent transactions.